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 than80 nm. The normal directions of the(001)B190twin planes are all within the range of 45° from the axial direction of the wire. The angle between twin crystals(1“11)Mand(111)Tof the SIM is also found to be smaller than that of thermally induced martensite in nanocrystalline NiTi. 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) B190twin planes are all within the range of 45 ° from the axial direction of the wire. The angle between twin crystals (1 ”11) Mand ( 111) Tof the SIM is also found to be smaller than that of thermally induced martensite in nanocrystalline NiTi.