A critical rule without free parameters to estimate the size-dependent enhancement or reduction of phase transition pressure Delta P-e(D) for phase transitions of nanocrystals is established where D denotes the size of nanocrystals. Delta P-e(D) function is determined by D, surface energy gamma, surface stress f and gram-atom volume V-g of the two transition phases. Also, the individual contributions of gamma and f on Delta P-e(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. (C) 2014 Elsevier B.V. All rights reserved.