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

    <schemaVersion>X0304</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2006-06-26</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0001042017</issuerCik>
        <issuerName>FRMO CORP</issuerName>
        <issuerTradingSymbol>FRMO</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001260568</rptOwnerCik>
            <rptOwnerName>Goldstein Lawrence J</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>1865 PALMER AVE 108</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>LARCHMONT</rptOwnerCity>
            <rptOwnerState>NY</rptOwnerState>
            <rptOwnerZipCode>10538</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>FRMO Corp.</value>
            </securityTitle>
            <transactionDate>
                <value>2011-11-15</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>P</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>28709</value>
                    <footnoteId id="F1"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>2.05</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>2768255</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>FRMO Corp.</value>
            </securityTitle>
            <transactionDate>
                <value>2011-11-15</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>S</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>28709</value>
                    <footnoteId id="F2"/>
                    <footnoteId id="F3"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>2.05</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>2768255</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <footnotes>
        <footnote id="F1">Santa Monica Partners, L.P. acquired 28,709 shares from its sister fund, Santa Monica Partners II, L.P. in a cross trade transaction.</footnote>
        <footnote id="F2">Santa Monica Partners II, L.P.  sold 28,709 shares to its sister fund, Santa Monica Partners, L.P. in a cross trade transaction.</footnote>
        <footnote id="F3">Ownership Breakdown: Lawrence J. Goldstein beneficially directly owns 1,509,050 shares, 2,500 shares in the Lawrence J. Goldstein IRA and 550 shares in the Lawrence J. Goldstein PSP. As President of SMP Asset Management, LLC, which is the General Partner of Santa Monica Partners, L.P., Mr. Goldstein beneficially owns indirectly 734,569 shares in Santa Monica Partners. As the General Partner of La'Dadande Limited Partnership, Mr. Goldstein beneficially owns 518,586 shares. Mr. Goldstein disclaims beneficial ownership of 3,000 shares held of record by the Barbara J. &amp; Lawrence J. Goldstein Do Good Foundation, Inc.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>Lawrence J. Goldstein</signatureName>
        <signatureDate>2011-11-15</signatureDate>
    </ownerSignature>
</ownershipDocument>
