<SUBMISSION>
<ACCESSION-NUMBER>0001140361-06-016453
<TYPE>3
<PUBLIC-DOCUMENT-COUNT>1
<PERIOD>20060615
<FILING-DATE>20061115
<DATE-OF-FILING-DATE-CHANGE>20061115
<ISSUER>
<COMPANY-DATA>
<CONFORMED-NAME>CLEAN DIESEL TECHNOLOGIES INC
<CIK>0000949428
<ASSIGNED-SIC>2810
<IRS-NUMBER>061393453
<STATE-OF-INCORPORATION>DE
<FISCAL-YEAR-END>1231
</COMPANY-DATA>
<BUSINESS-ADDRESS>
<STREET1>300 ATLANTIC ST
<STREET2>STE 702
<CITY>STAMFORD
<STATE>CT
<ZIP>06901
<PHONE>2033277050
</BUSINESS-ADDRESS>
<MAIL-ADDRESS>
<STREET1>300 ATLANTIC ST
<STREET2>STE 702
<CITY>STAMFORD
<STATE>CT
<ZIP>06901
</MAIL-ADDRESS>
</ISSUER>
<REPORTING-OWNER>
<OWNER-DATA>
<CONFORMED-NAME>MERRION DAVID F
<CIK>0001277525
</OWNER-DATA>
<FILING-VALUES>
<FORM-TYPE>3
<ACT>34
<FILE-NUMBER>000-27432
<FILM-NUMBER>061218150
</FILING-VALUES>
<BUSINESS-ADDRESS>
<STREET1>4522 LAKESHORE COURT
<CITY>BRIGHTON
<STATE>MI
<ZIP>48116
<PHONE>8102296643
</BUSINESS-ADDRESS>
</REPORTING-OWNER>
<DOCUMENT>
<TYPE>3
<SEQUENCE>1
<FILENAME>doc1.xml
<DESCRIPTION>FORM 3
<TEXT>
<XML>
<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0202</schemaVersion>

    <documentType>3</documentType>

    <periodOfReport>2006-06-15</periodOfReport>

    <noSecuritiesOwned>1</noSecuritiesOwned>

    <issuer>
        <issuerCik>0000949428</issuerCik>
        <issuerName>CLEAN DIESEL TECHNOLOGIES INC</issuerName>
        <issuerTradingSymbol>CDTI</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001277525</rptOwnerCik>
            <rptOwnerName>MERRION DAVID F</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O CLEAN DIESEL TECHNOLOGIES, INC.</rptOwnerStreet1>
            <rptOwnerStreet2>300 ATLANTIC ST., STE. 702</rptOwnerStreet2>
            <rptOwnerCity>STAMFORD</rptOwnerCity>
            <rptOwnerState>CT</rptOwnerState>
            <rptOwnerZipCode>06901</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle></officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>David F. Merrion</signatureName>
        <signatureDate>2006-11-15</signatureDate>
    </ownerSignature>
</ownershipDocument>
</XML>
</TEXT>
</DOCUMENT>
</SUBMISSION>
