<?xml version="1.0" encoding="UTF-8"?>
<edgarSubmission xmlns:ns1="http://www.sec.gov/edgar/common" xmlns="http://www.sec.gov/edgar/thirteenffiler">
  <schemaVersion>X0202</schemaVersion>
  <headerData>
    <submissionType>13F-HR</submissionType>
    <filerInfo>
      <liveTestFlag>LIVE</liveTestFlag>
      <flags>
        <confirmingCopyFlag>false</confirmingCopyFlag>
        <returnCopyFlag>false</returnCopyFlag>
        <overrideInternetFlag>false</overrideInternetFlag>
      </flags>
      <filer>
        <credentials>
          <cik>0000887818</cik>
          <ccc>XXXXXXXX</ccc>
        </credentials>
      </filer>
      <periodOfReport>06-30-2024</periodOfReport>
    </filerInfo>
  </headerData>
  <formData>
    <coverPage>
      <reportCalendarOrQuarter>06-30-2024</reportCalendarOrQuarter>
      <isAmendment>false</isAmendment>
      <filingManager>
        <name>FACTORY MUTUAL INSURANCE CO</name>
        <address>
          <ns1:street1>404 WYMAN STREET</ns1:street1>
          <ns1:street2>SUITE 390</ns1:street2>
          <ns1:city>WALTHAM</ns1:city>
          <ns1:stateOrCountry>MA</ns1:stateOrCountry>
          <ns1:zipCode>02451</ns1:zipCode>
        </address>
      </filingManager>
      <reportType>13F HOLDINGS REPORT</reportType>
      <form13FFileNumber>028-03189</form13FFileNumber>
      <provideInfoForInstruction5>N</provideInfoForInstruction5>
    </coverPage>
    <signatureBlock>
      <name>Sanjay Chawla</name>
      <title>Chief Investment Officer</title>
      <phone>617-243-7126</phone>
      <signature>/s/ Sanjay Chawla</signature>
      <city>Waltham</city>
      <stateOrCountry>MA</stateOrCountry>
      <signatureDate>07-22-2024</signatureDate>
    </signatureBlock>
    <summaryPage>
      <otherIncludedManagersCount>0</otherIncludedManagersCount>
      <tableEntryTotal>170</tableEntryTotal>
      <tableValueTotal>8778141622</tableValueTotal>
      <isConfidentialOmitted>false</isConfidentialOmitted>
    </summaryPage>
  </formData>
</edgarSubmission>
