Rare　earth　tungsten　cathodes　have　higher　emission　performance　than　pure　tungsten　cathodes.　In　this　paper,　lanthanum-tungsten　(La-W)　cathode　was　selected　to　research　its　microstructure　and　the　reasons　for　the　mi-grations　of　rare　earth　to　the　surface.　Scanning　electron　microscopy　(SEM)　and　transmission　electron　microscopy　(TEM)　were　used　to　characterize　the　microstructure　of　La-W　cathode　in　different　states.　Our　study　found　that　the　phase　in　bulk　existed　in　the　form　of　La2O3　and　had　a　clear　phase　interface　with　the　tungsten　matrix,　which　was　semi-coherent　or　incoherent　interfacial　relationship.　After　the　film　was　made,　the　boat-like　phase　would　form　nanocrystals,　and　the　phase　boundary　would　form　an　atomic　disorder　layer　of　4-5　nm,　forming　an　inco-herent　interface　with　a　higher　mismatch,　which　would　make　the　phase　more　easily　separated　from　the　matrix.　This　had　similar　characteristics　to　the　thermal　effect　of　the　bulk.　Thus,　the　mechanism　by　which　the　second　phase　migrated　to　the　surface　at　increasing　temperature　was　revealed.　Moreover,　it　was　found　that　the　migration　was　diffused　to　the　surface　along　the　grain　boundaries.