In　this　work,　the　nanostructured　Yb2O3-Yb2SiO5/mullite/Si　environmental　barrier　coatings　(EBCs)　were　designed　and　prepared　by　atmospheric　plasma　spraying.　The　water　vapor　corrosion　behavior　of　coatings　was　investigated　at　a　temperature　of　1350　degrees　C　for　durations　ranging　from　100　to　500　h,　and　the　influence　of　phase　compositions,　microstructure　on　this　behavior　was　systematically　investigated.　The　results　indicate　that　phase　transformation　occurs　during　the　water　vapor　corrosion　process,　and　severe　corrosion　takes　place　at　the　Yb2O3-Yb2SiO5　top　coating　and　the　mullite　intermediate　layer.　Obvious　diffusion　reaction　between　Yb2O3-Yb2SiO5　and　mullite　layers　occurred　without　water　vapor,　while　the　diffusion　reaction　cannot　be　observed　under　water　vapor.　The　water　vapor　corrosion　mechanism　of　EBCs　is　mainly　attributed　to　residual　thermal　mismatch　stresses,　phase　transformation　resulting　from　sintering　of　Yb2O3-Yb2SiO5,　as　well　as　the　formation　of　TGO　by　oxidation.