Abstract: For Ti-Ni alloy it is exceedingly difficult to prepare an element that has a desired and defined hysteresis temperature for its preparing process is not pedect at present. Because the mechanism of transformation in Ti-Ni has not been fully clarified, it is far more exceedingly difficult to explain the hysteresis temperature only by using the traditional metal physics techniques., But positron annihilation seems to be a suitable method for it. The positron annihilation measurements showthat the hysteresis temperature may be controlled mainly by the relative fraction of the defectscontained in a two-way shape memory element.