The　monofilamentary　MgB2/Fe　superconducting　tapes　were　fabricated　by　an　ex-situ　PIT　method.　The　quantitative　phase　analysis　was　carried　out　using　X-ray　diffraction　and　differential　thermal　analysis　on　short　samples　heat　treated　at　700,　850　and　920°C　in　vacuum,　respectively.　The　reaction　mechanism　of　MgB2　core　during　the　process　of　the　heat　treatment　was　discussed.　The　impurity　phase　of　MgO　was　found　in　all　MgB2　cores　and　the　largest　amount　was　detected　near　the　Fe　sheath　of　the　sample　annealed　at　the　highest　temperature.　MgB4　appeares　in　the　inner　part　of　MgB2　core,　which　is　possibly　formed　by　the　decomposition　of　MgB2.　A　reaction　layer　(Fe2B)　is　observed　between　MgB2　core　and　Fe　sheath,　which　is　decomposed　by　Fe23B6.　The　result　indicates　that　the　presence　of　the　oxygen　inside　sheath　material　is　the　main　obstacle　to　the　further　improvement　of　the　phase　purity　and　homogeneity　of　the　ex-situ　PIT　MgB2/Fe　tapes.