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

    <schemaVersion>X0303</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</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>2010-04-22</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>S</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>5800</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>3.25</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>2735569</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>FRMO Corp.</value>
            </securityTitle>
            <transactionDate>
                <value>2010-04-22</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>P</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>5800</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>3.25</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>2741369</value>
                    <footnoteId id="F1"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <footnotes>
        <footnote id="F1">Ownership Breakdown: Lawrence J. Goldstein beneficially owns directly 2,000,750 shares in his name, 2,500 shares in the name of Lawrence J. Goldstein, IRA and 550 shares in the name of Lawrence J. Goldstein PSP. As President of SMP Asset Management, LLC, which is General Partner of Santa Monica Partners, L.P., and of Santa Monica Partners Asset Management, LLC, which is a General Partner of Santa Monica Partners II, L.P., Mr. Goldstein beneficially owns indirectly 705,860 shares in Santa Monica Partners and 28,709 shares in Santa Monica Partners II, L.P. 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>2010-04-22</signatureDate>
    </ownerSignature>
</ownershipDocument>
