Submicron-grained Ti-50.9at%Ni alloy was prepared by Equal Channel Angular Extrusion (ECAE) at medium temperature(500°C). Microstructure evolution and phase transformations of Ti-50.9at%Ni alloy after ECAE processes at medium temperature have been investigated by optical microscope, TEM and DSC tests. It is found that the initial coarse grains of Ti-50.9at%Ni alloy were refined into submicron grains, smaller than 0.5 um in size, after eight passes ECAE at 500°C. Martensitic transformation temperatures decreased remarkably after one pass ECAE and more gently with increasing the pass number of ECAE. R phase transformation was stimulated after ECAE processes and underwent within a larger temperature range with an ambiguous thermal peak. The reverse R phase and reverse martensitic transformations, during heating, were separated eventually after eight passes ECAE.