A critical rule without free parameters to estimate the size-dependent enhancement or reduction of phase transition pressure ΔPe(D) for phase transitions of nanocrystals is established where D denotes the size of nanocrystals. ΔPe(D) function is determined by D, surface energy γ, surface stress f and gram-atom volume Vg of the two transition phases. Also, the individual contributions of γ and f on ΔPe(D) can be separated. Our model predictions agree well with the corresponding experimental and theoretical results. This model is of fundamental significance in understanding the essence of size-dependent pressure induced phase transition and guiding the experimental scientists to estimate their results.