It is generally considered that martensite start (Ms) temperature is decreased after plastic deformation of austenite if the previously applied load is retrieved. This can be explained by the dislocation stabilization mechanism. The present work performed systematic deformation and dilatometer experiments to investigate the effect of plastic deformation on Ms temperature. It is found that Ms temperature first increases at small strain and then decreases at large strain. Since the dislocation stabilization mechanism can only predict the decrease of Ms temperature after plastic deformation, a new mechanism is thus proposed to describe this new interesting finding. That is, the increase of Ms temperature is due to the pile-up of geometrically necessary dislocations at austenite grain boundaries, while the decrease of Ms temperature is caused by the formation of subgrains in austenite grain interior.