The　high-cycle　fatigue　mechanisms　of　a　NiTi　shape　memory　alloy　(SMA)　under　three　different　mean　strains　were　investigated.　Cold-rolled　NiTi　SMA　specimens　were　heat　treated　at　600　degrees　C　for　30min　was　selected　to　perform　the　fatigue　test.　Three　samples　were　conducted　cyclic　deformation　up　to　2x10(5)　cycles　with　three　mean　strains　that　subjected　at　full　austenite,　total　stress-induced　martensite　(SIM)　and　austenite-martensite　coexist　status　with　the　same　strain　amplitude　of　0.25%.　The　cyclic　stress-strain　(CSS)　curves　show　different　cyclic　softening　and　hardening　at　different　mean　strains.　The　S-S　curves　also　reveal　that　cyclic　deformation　strain　mainly　occurred　in　martensite　phase　although　austenite　and　martensite　(A-M)　coexisted.　The　transmission　electron　microscopy　(TEM)　investigation　shows　that　slip　dislocations　with　habit　plane　and　basal　plane　dislocations　were　induced　during　the　cycling,　and　these　microstructure　evolutions　contribute　to　the　cyclic　softening　and　hardening.