The　microstructure　of　stress-induced　martensite　(SIM)　in　the　nanocrystalline　NiTi　alloy　was　investigated　by　means　of　transmission　electron　microscopy　(TEM).　The　result　shows　that　the　multi-variant　structure　of　the　martensite　is　suppressed　and　only　single-variant　martensitic　twins　form　after　tensile　deformation　when　the　grain　size　is　smaller　than　80　nm.　The　normal　directions　of　the　(001)(B19＇)　twin　planes　are　all　within　the　range　of　45A　degrees　from　the　axial　direction　of　the　wire.　The　angle　between　twin　crystals　()(M)　and　(111)(T)　of　the　SIM　is　also　found　to　be　smaller　than　that　of　thermally　induced　martensite　in　nanocrystalline　NiTi.