<?xml version="1.0"?>
<ownershipDocument>
    <schemaVersion>X0101</schemaVersion>

    <documentType>4</documentType><periodOfReport>2003-06-30</periodOfReport>
    <issuer>
        <issuerCik>0000716823</issuerCik>
        <issuerName>MILACRON INC</issuerName>
        <issuerTradingSymbol>MZ</issuerTradingSymbol></issuer><reportingOwner><reportingOwnerId>
            <rptOwnerCik>0001197861</rptOwnerCik>
            <rptOwnerName>STEGER JOSEPH A</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>  </rptOwnerStreet1>
            <rptOwnerStreet2>  </rptOwnerStreet2>
            <rptOwnerCity>  </rptOwnerCity>
            <rptOwnerState>  </rptOwnerState>
            <rptOwnerZipCode>  </rptOwnerZipCode>
        </reportingOwnerAddress>
            <reportingOwnerRelationship>
<isDirector>1</isDirector><isOfficer>0</isOfficer><isTenPercentOwner>0</isTenPercentOwner><isOther>0</isOther></reportingOwnerRelationship></reportingOwner><derivativeSecurity><securityTitle><value>Phantom Stock Units</value><footnoteId id="F1"/></securityTitle><conversionOrExercisePrice><value>4.995</value></conversionOrExercisePrice><transactionDate><value>2003-06-30</value></transactionDate><transactionCoding><transactionFormType>4</transactionFormType><transactionCode>A</transactionCode><equitySwapInvolved>0</equitySwapInvolved></transactionCoding><transactionAmounts><transactionShares><value>500.5</value></transactionShares><transactionValue><value>0</value></transactionValue><transactionAcquiredDisposedCode><value>A</value></transactionAcquiredDisposedCode></transactionAmounts><exerciseDate><value>1988-08-08</value><footnoteId id="F2"/></exerciseDate><expirationDate><value>1988-08-08</value></expirationDate><underlyingSecurity><underlyingSecurityTitle><value>Common Stock</value></underlyingSecurityTitle><underlyingSecurityShares><value>500.5</value></underlyingSecurityShares></underlyingSecurity><postTransactionAmounts><sharesOwnedFollowingTransaction><value>10887.02</value></sharesOwnedFollowingTransaction></postTransactionAmounts><ownershipNature><directOrIndirectOwnership><value>D</value></directOrIndirectOwnership><natureOfOwnership><value/></natureOfOwnership></ownershipNature></derivativeSecurity><footnotes><footnote id="F1">The reporting person also directly owns stock options to purchase 17,000 shares of MZ common stock, and directly beneficially owns 1,646 shares of MZ common stock and 1,500 restricted shares of MZ common stock.</footnote><footnote id="F2">Acquisition of phantom stock units under the company's deferred compensation plan for non-employee directors.  Benefits are payable upon the earlier of termination of services as a director or death.</footnote></footnotes><ownerSignature><signatureName>Walter S. Wood</signatureName><signatureDate>2003-07-01</signatureDate></ownerSignature></ownershipDocument>

