TiNi shape memory alloy exhibits excellent shape memory effect, superelasticity, corrosion resistance and biocompatibility, and is one of the most important engineering materials. Area of application has been rapidly expanding, especially in the field of medical devices, e.g., orthodontic wires, stents, guide wires, etc. Another unique characteristics of this material is that it undergoes crystalline-to-amorphous transformation in solid state. Amorphization can be induced by particle beam irradiation (electron, neutron and ions) 1, 2), and by severe plastic deformation, such as, cold rolling (CR), shot peening(SP) and high pressure torsion (HPT). Present paper describes the changes in microstructures during severe plastic deformation by HPT as well as martensitic transformation, and mechanical properties of amorphous/nanocrystalline TiNi. We investigated the effect of HPT deformation and post-deformation aging on the microstructures, martensitic transformation and mechanical properties were described. It was revealed that stress-induced B19' phase reverts to B2 phase by deformation, and local amorphization starts at a relatively early stage of deformation. Post-deformation aging resulted in the formation of nanocrystalline B2 phase. Nano-scale grain size stabilizes the B2 phase and suppress the formation of B19' martensite while it stabilize the R phase to some extent. Effect of elastic energy and irreversible energy on transformation behavior in nanocrystalline TiNi was discussed. The nanocrystalline TiNi exhibits very high value of microhardness which can be beneficial to increase shape recovery stress and a wider temperature window for superelasticity. The nanostrcutured TiNi exhibits high stregth and high elastic modulus compared to conventional TiNi. These properteies can be useful for medical devices, e.g., stents and guide wires. Properties of amorphous/nanocrystalline TiNi wires is also under investigation.