<?xml version="1.0"?>
<ownershipDocument>
    <schemaVersion>X0201</schemaVersion>

    <documentType>4</documentType><periodOfReport>2003-09-30</periodOfReport>
    <issuer>
        <issuerCik>0000098362</issuerCik>
        <issuerName>TIMKEN CO</issuerName>
        <issuerTradingSymbol>TKR</issuerTradingSymbol></issuer><reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0000897930</rptOwnerCik>
            <rptOwnerName>PRECOURT JAY A</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>1835 DUEBER AVE. S. W.</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>CANTON</rptOwnerCity>
            <rptOwnerState>OH</rptOwnerState>
            <rptOwnerZipCode>44706</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
            <reportingOwnerRelationship>
<isDirector>1</isDirector><isOfficer>0</isOfficer><isTenPercentOwner>0</isTenPercentOwner><isOther>0</isOther></reportingOwnerRelationship></reportingOwner><derivativeTable><derivativeTransaction><securityTitle><value>Phantom Stock</value></securityTitle><conversionOrExercisePrice><value>0</value><footnoteId id="F1"/></conversionOrExercisePrice><transactionDate><value>2003-09-30</value></transactionDate><transactionCoding><transactionFormType>4</transactionFormType><transactionCode>A</transactionCode><equitySwapInvolved>0</equitySwapInvolved></transactionCoding><transactionAmounts><transactionShares><value>1061.74</value></transactionShares><transactionPricePerShare><value>15.305</value></transactionPricePerShare><transactionAcquiredDisposedCode><value>A</value></transactionAcquiredDisposedCode></transactionAmounts><exerciseDate><value>2003-09-30</value></exerciseDate><expirationDate><value>1988-08-08</value><footnoteId id="F2"/></expirationDate><underlyingSecurity><underlyingSecurityTitle><value>Common Stock</value></underlyingSecurityTitle><underlyingSecurityShares><value>1061.74</value></underlyingSecurityShares></underlyingSecurity><postTransactionAmounts><sharesOwnedFollowingTransaction><value>6148.69</value><footnoteId id="F3"/></sharesOwnedFollowingTransaction></postTransactionAmounts><ownershipNature><directOrIndirectOwnership><value>D</value></directOrIndirectOwnership><natureOfOwnership><value/></natureOfOwnership></ownershipNature></derivativeTransaction></derivativeTable><footnotes><footnote id="F1">The Security converts to common stock on a one for one basis</footnote><footnote id="F2">The units are to be settled within 1 year after directorship ceases</footnote><footnote id="F3">Includes 35.21 phantom stock units acquired as deemed dividends under The Timken Company Director Deferred Compensation plan on 09/03/2003</footnote></footnotes><remarks/><ownerSignature><signatureName>Scott A. Scherff,  Attorney in fact</signatureName><signatureDate>2003-10-01</signatureDate></ownerSignature></ownershipDocument>

