Nanocrystals consisting of small crystallites of diameter 1-100 nm, often have novel physical and chemical properties, differing from those of the corresponding bulk materials. The issue of the effects of crystallite size on the structural stability in these nanocrystals is of considerable interest from a fundamental viewpoint, and also with respect to the applicability of these materials. How will the relative stability of different possible crystal structures change for nanocrystals with respect to bulk materials? One way to answer this question is to use pressure to force nanostructured materials to convert from one crystal structure to another. Here we report the grain-size effect on the phase transition induced by the pressure in various systems using in-situ high-pressure and high-temperature synchrotron radiation X-ray powder diffraction technique. We demonstrate that the grain-size effect on the structural stability in nanocrystals can be of either sign with respect to the change of the transition pressure, depending on the system under investigation. The effect will be further discussed from a thermodynamic theory. Three components: the volume collapses at the transition, the surface energy differences, and the internal energy differences, are identified to govern the change of transition pressure in nanocrystals. The competing processes can be applied to explain results reported in the literature and to uncover the major factors underlying the change of the transition pressure in nanocrystals as compared with the corresponding bulk material.