﻿<?xml version="1.0" encoding="utf-8"?>
<InstanceReport xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
  <Version>2.2.0.7</Version>
  <hasSegments>false</hasSegments>
  <ReportName>BASIS OF PRESENTATION</ReportName>
  <ReportLongName>1010 - Disclosure - BASIS OF PRESENTATION</ReportLongName>
  <DisplayLabelColumn>true</DisplayLabelColumn>
  <ShowElementNames>false</ShowElementNames>
  <RoundingOption />
  <HasEmbeddedReports>false</HasEmbeddedReports>
  <Columns>
    <Column>
      <LabelColumn>false</LabelColumn>
      <Id>1</Id>
      <Labels>
        <Label Id="1" Label="9 Months Ended" />
        <Label Id="2" Label="Sep. 30, 2010" />
      </Labels>
      <CurrencyCode>USD</CurrencyCode>
      <FootnoteIndexer />
      <hasSegments>false</hasSegments>
      <hasScenarios>false</hasScenarios>
      <Segments />
      <Scenarios />
      <Units>
        <Unit>
          <UnitID>USD</UnitID>
          <UnitType>Standard</UnitType>
          <StandardMeasure>
            <MeasureSchema>http://www.xbrl.org/2003/iso4217</MeasureSchema>
            <MeasureValue>USD</MeasureValue>
            <MeasureNamespace>iso4217</MeasureNamespace>
          </StandardMeasure>
          <Scale>0</Scale>
        </Unit>
        <Unit>
          <UnitID>USDPerShare</UnitID>
          <UnitType>Divide</UnitType>
          <NumeratorMeasure>
            <MeasureSchema>http://www.xbrl.org/2003/iso4217</MeasureSchema>
            <MeasureValue>USD</MeasureValue>
            <MeasureNamespace>iso4217</MeasureNamespace>
          </NumeratorMeasure>
          <DenominatorMeasure>
            <MeasureSchema>http://www.xbrl.org/2003/instance</MeasureSchema>
            <MeasureValue>shares</MeasureValue>
            <MeasureNamespace>xbrli</MeasureNamespace>
          </DenominatorMeasure>
          <Scale>0</Scale>
        </Unit>
        <Unit>
          <UnitID>Shares</UnitID>
          <UnitType>Standard</UnitType>
          <StandardMeasure>
            <MeasureSchema>http://www.xbrl.org/2003/instance</MeasureSchema>
            <MeasureValue>shares</MeasureValue>
            <MeasureNamespace>xbrli</MeasureNamespace>
          </StandardMeasure>
          <Scale>0</Scale>
        </Unit>
      </Units>
      <CurrencySymbol>$</CurrencySymbol>
    </Column>
  </Columns>
  <Rows>
    <Row>
      <Id>2</Id>
      <Label>BASIS OF PRESENTATION</Label>
      <Level>0</Level>
      <ElementName>pl_BasisOfPresentationDisclosureAbstract</ElementName>
      <ElementPrefix>pl</ElementPrefix>
      <IsBaseElement>false</IsBaseElement>
      <BalanceType>na</BalanceType>
      <PeriodType>duration</PeriodType>
      <ShortDefinition>No definition available.</ShortDefinition>
      <IsReportTitle>false</IsReportTitle>
      <IsSegmentTitle>false</IsSegmentTitle>
      <IsSubReportEnd>false</IsSubReportEnd>
      <IsCalendarTitle>false</IsCalendarTitle>
      <IsTuple>false</IsTuple>
      <IsAbstractGroupTitle>true</IsAbstractGroupTitle>
      <IsEquityPrevioslyReportedAsRow>false</IsEquityPrevioslyReportedAsRow>
      <IsEquityAdjustmentRow>false</IsEquityAdjustmentRow>
      <IsBeginningBalance>false</IsBeginningBalance>
      <IsEndingBalance>false</IsEndingBalance>
      <IsReverseSign>false</IsReverseSign>
      <PreferredLabelRole />
      <IsEPS>false</IsEPS>
      <FootnoteIndexer />
      <Cells>
        <Cell>
          <Id>1</Id>
          <ShowCurrencySymbol>false</ShowCurrencySymbol>
          <IsNumeric>false</IsNumeric>
          <IsRatio>false</IsRatio>
          <DisplayZeroAsNone>false</DisplayZeroAsNone>
          <NumericAmount>0</NumericAmount>
          <RoundedNumericAmount>0</RoundedNumericAmount>
          <NonNumbericText />
          <NonNumericTextHeader />
          <FootnoteIndexer />
          <hasSegments>false</hasSegments>
          <hasScenarios>false</hasScenarios>
          <DisplayDateInUSFormat>false</DisplayDateInUSFormat>
        </Cell>
      </Cells>
      <OriginalInstanceReportColumns />
      <ElementDataType>xbrli:stringItemType</ElementDataType>
      <SimpleDataType>string</SimpleDataType>
      <ElementDefenition>No definition available.</ElementDefenition>
      <IsTotalLabel>false</IsTotalLabel>
    </Row>
    <Row>
      <Id>3</Id>
      <Label>BASIS OF PRESENTATION</Label>
      <Level>1</Level>
      <ElementName>us-gaap_OrganizationConsolidationAndPresentationOfFinancialStatementsDisclosureTextBlock</ElementName>
      <ElementPrefix>us-gaap</ElementPrefix>
      <IsBaseElement>true</IsBaseElement>
      <BalanceType>na</BalanceType>
      <PeriodType>duration</PeriodType>
      <ShortDefinition>No definition available.</ShortDefinition>
      <IsReportTitle>false</IsReportTitle>
      <IsSegmentTitle>false</IsSegmentTitle>
      <IsSubReportEnd>false</IsSubReportEnd>
      <IsCalendarTitle>false</IsCalendarTitle>
      <IsTuple>false</IsTuple>
      <IsAbstractGroupTitle>false</IsAbstractGroupTitle>
      <IsEquityPrevioslyReportedAsRow>false</IsEquityPrevioslyReportedAsRow>
      <IsEquityAdjustmentRow>false</IsEquityAdjustmentRow>
      <IsBeginningBalance>false</IsBeginningBalance>
      <IsEndingBalance>false</IsEndingBalance>
      <IsReverseSign>false</IsReverseSign>
      <PreferredLabelRole>terselabel</PreferredLabelRole>
      <IsEPS>false</IsEPS>
      <FootnoteIndexer />
      <Cells>
        <Cell>
          <Id>1</Id>
          <ShowCurrencySymbol>false</ShowCurrencySymbol>
          <IsNumeric>false</IsNumeric>
          <IsRatio>false</IsRatio>
          <DisplayZeroAsNone>false</DisplayZeroAsNone>
          <NumericAmount>0</NumericAmount>
          <RoundedNumericAmount>0</RoundedNumericAmount>
          <NonNumbericText>&lt;table style="font-size:10pt; font-family:'Times New Roman',times,serif;"&gt;
&lt;tr&gt;
&lt;td&gt;
&lt;p style="MARGIN: 0in 0in 0pt"&gt;&lt;b&gt;&lt;font style="FONT-WEIGHT: bold; FONT-SIZE: 10pt; FONT-FAMILY: Times New Roman" size="2"&gt;1.&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp; BASIS OF PRESENTATION&lt;/font&gt;&lt;/b&gt;&lt;/p&gt;
&lt;p style="MARGIN: 0in 0in 0pt"&gt;&lt;font style="FONT-SIZE: 10pt; FONT-FAMILY: Times New Roman" size="2"&gt;&amp;nbsp;&lt;/font&gt;&lt;/p&gt;
&lt;p style="MARGIN: 0in 0in 0pt 0.5in; TEXT-INDENT: -0.5in"&gt;&lt;b&gt;&lt;font style="FONT-WEIGHT: bold; FONT-SIZE: 10pt; FONT-FAMILY: Times New Roman" size="2"&gt;Basis of Presentation&lt;/font&gt;&lt;/b&gt;&lt;/p&gt;
&lt;p style="MARGIN: 0in 0in 0pt 0.5in; TEXT-INDENT: -0.5in"&gt;&lt;font style="FONT-SIZE: 10pt; FONT-FAMILY: Times New Roman" size="2"&gt;&amp;nbsp;&lt;/font&gt;&lt;/p&gt;
&lt;p style="MARGIN: 0in 0in 0pt; TEXT-INDENT: 0.5in"&gt;&lt;font style="FONT-SIZE: 10pt; FONT-FAMILY: Times New Roman" size="2"&gt;The accompanying unaudited consolidated condensed financial statements of Protective Life Corporation and subsidiaries&amp;nbsp;(the&amp;nbsp;&amp;#147;Company&amp;#148;) have been prepared in accordance with accounting principles generally accepted in the United States of America&amp;nbsp;(&amp;#147;GAAP&amp;#148;) for interim financial information and with the instructions to Form&amp;nbsp;10-Q and Rule&amp;nbsp;10-01 of Regulation&amp;nbsp;S-X. Accordingly, they do not include all of the disclosures required by GAAP for complete financial statements. In the opinion of management, the accompanying financial statements reflect all adjustments (consisting only of normal recurring items) necessary for a fair statement of the results for the interim periods presented. Operating results for the three and nine months period ended September&amp;nbsp;30, 2010, are not necessarily indicative of the results that may be expected for the year ending December&amp;nbsp;31, 2010. The year-end consolidated condensed balance sheet data was derived from audited financial statements, but does not include all disclosures required by GAAP. For further information, refer to the consolidated financial statements and notes thereto included in the Company&amp;#146;s Annual Report on Form&amp;nbsp;10-K for the year ended December&amp;nbsp;31, 2009.&lt;/font&gt;&lt;/p&gt;
&lt;p style="MARGIN: 0in 0in 0pt; TEXT-INDENT: 0.5in"&gt;&lt;font style="FONT-SIZE: 10pt; FONT-FAMILY: Times New Roman" size="2"&gt;&amp;nbsp;&lt;/font&gt;&lt;/p&gt;
&lt;p style="MARGIN: 0in 0in 0pt; TEXT-INDENT: 0.5in"&gt;&lt;font style="FONT-SIZE: 10pt; FONT-FAMILY: Times New Roman" size="2"&gt;The operating results of companies in the insurance industry have historically been subject to significant fluctuations due to changing competition, economic conditions, interest rates, investment performance, insurance ratings, claims, persistency, and other factors.&lt;/font&gt;&lt;/p&gt;
&lt;p style="MARGIN: 0in 0in 0pt; TEXT-INDENT: 0.5in"&gt;&lt;font style="FONT-SIZE: 10pt; FONT-FAMILY: Times New Roman" size="2"&gt;&amp;nbsp;&lt;/font&gt;&lt;/p&gt;
&lt;p style="MARGIN: 0in 0in 0pt"&gt;&lt;b&gt;&lt;font style="FONT-WEIGHT: bold; FONT-SIZE: 10pt; FONT-FAMILY: Times New Roman" size="2"&gt;Reclassifications&lt;/font&gt;&lt;/b&gt;&lt;/p&gt;
&lt;p style="MARGIN: 0in 0in 0pt"&gt;&lt;font style="FONT-SIZE: 10pt; FONT-FAMILY: Times New Roman" size="2"&gt;&amp;nbsp;&lt;/font&gt;&lt;/p&gt;
&lt;p style="MARGIN: 0in 0in 0pt; TEXT-INDENT: 0.5in"&gt;&lt;font style="FONT-SIZE: 10pt; FONT-FAMILY: Times New Roman" size="2"&gt;Certain reclassifications have been made in the previously reported financial statements and accompanying notes to make the prior year amounts comparable to those of the current year. Such reclassifications had no effect on previously reported net income or shareowners&amp;#146; equity.&lt;/font&gt;&lt;/p&gt;
&lt;p style="MARGIN: 0in 0in 0pt"&gt;&lt;font style="FONT-SIZE: 10pt; FONT-FAMILY: Times New Roman" size="2"&gt;&amp;nbsp;&lt;/font&gt;&lt;/p&gt;
&lt;p style="MARGIN: 0in 0in 0pt 0.5in; TEXT-INDENT: -0.5in"&gt;&lt;b&gt;&lt;font style="FONT-WEIGHT: bold; FONT-SIZE: 10pt; FONT-FAMILY: Times New Roman" size="2"&gt;Entities Included&lt;/font&gt;&lt;/b&gt;&lt;/p&gt;
&lt;p style="MARGIN: 0in 0in 0pt 0.5in; TEXT-INDENT: -0.5in"&gt;&lt;font style="FONT-SIZE: 10pt; FONT-FAMILY: Times New Roman" size="2"&gt;&amp;nbsp;&lt;/font&gt;&lt;/p&gt;
&lt;p style="MARGIN: 0in 0in 0pt; TEXT-INDENT: 0.5in"&gt;&lt;font style="FONT-SIZE: 10pt; FONT-FAMILY: Times New Roman" size="2"&gt;The consolidated condensed financial statements include the accounts of Protective Life Corporation and subsidiaries and its affiliate companies in which the Company holds a majority voting or economic interest. Intercompany balances and transactions have been eliminated.&lt;/font&gt;&lt;/p&gt;&lt;/td&gt;&lt;/tr&gt;&lt;/table&gt;
</NonNumbericText>
          <NonNumericTextHeader>1.&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp; BASIS OF PRESENTATION
&amp;nbsp;
Basis of Presentation
&amp;nbsp;
The accompanying unaudited</NonNumericTextHeader>
          <FootnoteIndexer />
          <hasSegments>false</hasSegments>
          <hasScenarios>false</hasScenarios>
          <DisplayDateInUSFormat>false</DisplayDateInUSFormat>
        </Cell>
      </Cells>
      <OriginalInstanceReportColumns />
      <ElementDataType>us-types:textBlockItemType</ElementDataType>
      <SimpleDataType>textblock</SimpleDataType>
      <ElementDefenition>Description containing the entire organization, consolidation and basis of presentation of financial statements disclosure. May be provided in more than one note to the financial statements, as long as users are provided with an understanding of (1) the significant judgments and assumptions made by an enterprise in determining whether it must consolidate a VIE and/or disclose information about its involvement with a VIE, (2) the nature of restrictions on a consolidated VIE's assets reported by an enterprise in its statement of financial position, including the carrying amounts of such assets, (3) the nature of, and changes in, the risks associated with an enterprise's involvement with the VIE, and (4) how an enterprise's involvement with the VIE affects the enterprise's financial position, financial performance, and cash flows.  Describes procedure if disclosures are provided in more than one note to the financial statements.</ElementDefenition>
      <ElementReferences>Reference 1: http://www.xbrl.org/2003/role/presentationRef
 -Publisher FASB
 -Name FASB Staff Position (FSP)
 -Number FAS140-4 and FIN46(R)-8
 -Paragraph 8, C1, C7

Reference 2: http://www.xbrl.org/2003/role/presentationRef
 -Publisher AICPA
 -Name Accounting Research Bulletin (ARB)
 -Number 51
 -Paragraph 2-6

Reference 3: http://www.xbrl.org/2003/role/presentationRef
 -Publisher AICPA
 -Name Statement of Position (SOP)
 -Number 94-6
 -Paragraph 10

Reference 4: http://www.xbrl.org/2003/role/presentationRef
 -Publisher FASB
 -Name FASB Interpretation (FIN)
 -Number 46R
 -Paragraph 4, 14, 15

</ElementReferences>
      <IsTotalLabel>false</IsTotalLabel>
    </Row>
  </Rows>
  <Footnotes />
  <NumberOfCols>1</NumberOfCols>
  <NumberOfRows>2</NumberOfRows>
  <HasScenarios>false</HasScenarios>
  <MonetaryRoundingLevel>UnKnown</MonetaryRoundingLevel>
  <SharesRoundingLevel>UnKnown</SharesRoundingLevel>
  <PerShareRoundingLevel>UnKnown</PerShareRoundingLevel>
  <HasPureData>false</HasPureData>
  <SharesShouldBeRounded>true</SharesShouldBeRounded>
</InstanceReport>
