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

    <schemaVersion>X0303</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2008-10-20</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0000080035</issuerCik>
        <issuerName>PREFORMED LINE PRODUCTS CO</issuerName>
        <issuerTradingSymbol>PLPC</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001315040</rptOwnerCik>
            <rptOwnerName>McKenna Dennis F</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>P.O. BOX 91129</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>CLEVELAND</rptOwnerCity>
            <rptOwnerState>OH</rptOwnerState>
            <rptOwnerZipCode>44101</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>V.P. - Global Business Dev.</officerTitle>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common shares, $2 par value</value>
            </securityTitle>
            <transactionDate>
                <value>2008-10-20</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>13</value>
                    <footnoteId id="F1"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>4335</value>
                    <footnoteId id="F2"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <derivativeTable>
        <derivativeHolding>
            <securityTitle>
                <value>Employee stock option (right to buy)</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>15.125</value>
            </conversionOrExercisePrice>
            <exerciseDate>
                <value>2001-02-16</value>
            </exerciseDate>
            <expirationDate>
                <value>2010-02-15</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common shares, $2 par value</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>1300</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>1300</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeHolding>
        <derivativeHolding>
            <securityTitle>
                <value>Employee stock option (right to buy)</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>22.1</value>
            </conversionOrExercisePrice>
            <exerciseDate>
                <value>2005-07-28</value>
            </exerciseDate>
            <expirationDate>
                <value>2014-07-27</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common shares, $2 par value</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>5000</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>5000</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeHolding>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">These restricted Common shares granted to the Reporting Person represent the dividend on previously granted shares pursuant to the Company's Long Term Incentive Plan of 2008.  Provided that the Reporting Person has not voluntarily terminated his employment, 2 shares vest and will no longer be subject to risk of forfeiture on 8/29/2011, and depending continued employment and on Company performance 11 shares vest and will no longer be subject to risk of forfeiture on 12/31/2010.</footnote>
        <footnote id="F2">Includes 3,925 restricted Common shares  granted to the Reporting Person pursuant to the Company's Long Term Incentive Plan of 2008.  Provided that the Reporting Person has not voluntarily terminated his employment, 784 shares vest and will no longer be subject to risk of forfeiture on 8/29/2011, and depending continued employment and on Company performance 3,141 shares vest and will no longer be subject to risk of forfeiture on 12/31/2010.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>/s/Eric R. Graef, by power of attorney</signatureName>
        <signatureDate>2008-10-20</signatureDate>
    </ownerSignature>
</ownershipDocument>
