Abstract: A number of specimens of Ti-51Ni (at. pct) allOy showing reversible shape memory effect (RSME) have been measured as to reversibility and S-lineshape parameter of positron annihilation. It is found that the Ti-51Ni specimens showing good memory effect contain Structural defects. Not all specimens including Structural defects (or Structural impedrition) are able to exhibit good memory effect. The structural defects have to be appropriate in density neither too much nor too little. With no or too much Structural defects a good RSME could not take place. It is fully as important that the Structural defects exist as it is for the structural defects to be appropriate in density in order to obtain a good RSME.