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

    <schemaVersion>X0202</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2006-02-14</periodOfReport>

    <issuer>
        <issuerCik>0000094049</issuerCik>
        <issuerName>STEPAN CO</issuerName>
        <issuerTradingSymbol>SCL</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001229309</rptOwnerCik>
            <rptOwnerName>CADIEUX ROBERT D</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>22 W. FRONTAGE ROAD</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>NORTHFIELD</rptOwnerCity>
            <rptOwnerState>IL</rptOwnerState>
            <rptOwnerZipCode>60093</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
        </reportingOwnerRelationship>
    </reportingOwner>

    <derivativeTable>
        <derivativeTransaction>
            <securityTitle>
                <value>Directors Deferred Compensation Plan</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>26.25</value>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2006-02-14</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>J</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>571</value>
                    <footnoteId id="F1"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>26.25</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <value>1988-08-08</value>
            </exerciseDate>
            <expirationDate>
                <value>1988-08-08</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>571</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>28420</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </derivativeTransaction>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">On February 14, 2006, 571 shares were added to the Directors Deferred Compensation Plan account as per the provisions in the First Amendment to the Stepan Company Directors Deferred Compensation Plan.  The 571 shares shall be paid to the director upon termination of service on the Board of Directors and shall be paid only in shares of Stepan Company Common Stock.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>Kathleen M. Owens, by Power of Attorney for Robert D. Cadieux</signatureName>
        <signatureDate>2006-02-16</signatureDate>
    </ownerSignature>
</ownershipDocument>
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X0202
4
2006-02-14
0000094049
STEPAN CO
SCL
0001229309
CADIEUX ROBERT D
22 W. FRONTAGE ROAD
NORTHFIELD
IL
60093
1
0
0
0
Directors Deferred Compensation Plan
26.25
2006-02-14
4
J
0
571
26.25
A
1988-08-08
1988-08-08
Common Stock
571
28420
D
On February 14, 2006, 571 shares were added to the Directors Deferred Compensation Plan account as per the provisions in the First Amendment to the Stepan Company Directors Deferred Compensation Plan.  The 571 shares shall be paid to the director upon termination of service on the Board of Directors and shall be paid only in shares of Stepan Company Common Stock.
Kathleen M. Owens, by Power of Attorney for Robert D. Cadieux
2006-02-16