Abstract: Recent development of grain－size effect on phase transformations of nanocrystals induced by pressure is reported. A thermodynamic theory is presented and three components: the ratio of volume collapses, the surface energy differences, and the internal energy differences, governing the change of transition pressure in nanocrystals were uncovered. They can be used to explain the results reported in the literature and to identify the main factor to the change of the transition pressure in nanocrystals. We demonstrated that the grain－size effect on the structural stability in nanocrystals with respect to transition pressure can be of either sign, depending on the system under investigation.