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

    <documentType>4</documentType><periodOfReport>2003-08-29</periodOfReport>
    <issuer>
        <issuerCik>0000018230</issuerCik>
        <issuerName>CATERPILLAR INC</issuerName>
        <issuerTradingSymbol>CAT</issuerTradingSymbol></issuer><reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001172570</rptOwnerCik>
            <rptOwnerName>POWELL CHARLES D</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>BERKELEY SQUARE HOUSE, 4TH FLOOR</rptOwnerStreet1>
            <rptOwnerStreet2>BERKELEY SQUARE</rptOwnerStreet2>
            <rptOwnerCity>LONDON ENGLAND</rptOwnerCity>
            <rptOwnerState></rptOwnerState>
            <rptOwnerZipCode>W1X 5PN</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
            <reportingOwnerRelationship>
<isDirector>1</isDirector><isOfficer>0</isOfficer><isTenPercentOwner>0</isTenPercentOwner><isOther>0</isOther></reportingOwnerRelationship></reportingOwner><nonDerivativeTable><nonDerivativeTransaction><securityTitle><value>Common</value></securityTitle><transactionDate><value>2003-08-29</value></transactionDate><transactionCoding><transactionFormType>4</transactionFormType><transactionCode>A</transactionCode><equitySwapInvolved>0</equitySwapInvolved><footnoteId id="F1"/></transactionCoding><transactionAmounts><transactionShares><value>26</value></transactionShares><transactionPricePerShare><value>71.515</value></transactionPricePerShare><transactionAcquiredDisposedCode><value>A</value></transactionAcquiredDisposedCode></transactionAmounts><postTransactionAmounts><sharesOwnedFollowingTransaction><value>1647</value><footnoteId id="F2"/></sharesOwnedFollowingTransaction></postTransactionAmounts><ownershipNature><directOrIndirectOwnership><value>D</value></directOrIndirectOwnership><natureOfOwnership><value/></natureOfOwnership></ownershipNature></nonDerivativeTransaction></nonDerivativeTable><footnotes><footnote id="F1">Stock received in lieu of director's fees.</footnote><footnote id="F2">Included in this amount are 32 shares in dividend reinvestment.</footnote></footnotes><remarks/><ownerSignature><signatureName>Charles D. Powell; L.J. Huxtable, POA</signatureName><signatureDate>2003-09-02</signatureDate></ownerSignature></ownershipDocument>

