<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0303</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2008-07-17</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0000009521</issuerCik>
        <issuerName>BANCROFT FUND LTD</issuerName>
        <issuerTradingSymbol>BCV</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001014776</rptOwnerCik>
            <rptOwnerName>DINSMORE CAPITAL MANAGEMENT CO/NJ</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>65 MADISON AVE</rptOwnerStreet1>
            <rptOwnerStreet2>SUITE 550</rptOwnerStreet2>
            <rptOwnerCity>MORRISTOWN</rptOwnerCity>
            <rptOwnerState>NJ</rptOwnerState>
            <rptOwnerZipCode>07960</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>1</isOther>
            <otherText>Investment adviser</otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Shares of Beneficial Interest</value>
            </securityTitle>
            <transactionDate>
                <value>2008-07-17</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>J</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
                <footnoteId id="F1"/>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>2797</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>18.1</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>2003</value>
                    <footnoteId id="F2"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <footnotes>
        <footnote id="F1">This report is filed to report the disposition of 2,797 shares, transferred to Thomas H. Dinsmore and Jane D. O'Keeffe as bonuses for services rendered as of the adviser's fiscal year end, June 30, 2008.</footnote>
        <footnote id="F2">Includes 1,531 shares acquired on various dates in fiscal years 2003 through 2008 under the BCV dividend reinvestment plan.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>/s/ Gary I. Levine, attorney-in-fact</signatureName>
        <signatureDate>2008-07-21</signatureDate>
    </ownerSignature>
</ownershipDocument>
