Nanocrystalline (NC) Ti_49.4Ni_50.6 alloy with the grain size of 20-100 nm has been successfully produced using high pressure torsion (HPT) and annealing. The ultra fine grained (UFG) structure with the grain size of 300 nm in Ti_49.4Ni_50.6 alloy has been produced using equal channel angular pressing (ECAP). The mechanisms of deformation behavior of the NC and UFG TiNi have been determined. The features of microstructure, martensitic transformation and deformation behavior of the NC and UFG alloy at room and elevated temperatures have been studied in details. The NC TiNi exhibits unusual enhanced strength. The UFG TiNi exhibits unusual enhanced ductility and high strength. The effects of the grain size on mechanical properties of the alloy are discussed.