Based on the boron nitride thermodynamic equilibrium phase diagram, a thermodynamic analysis in nanoscale was proposed, with respect to the effect of nanosize-induced additional pressure on Gibbs free energy of critical nuclei of cubic boron nitride (c-BN), to elucidate nucleation of c-BN in a high-pressure and high-temperature (HPHT) supercritical fluid system. Notably, our analysis showed that the effect of surface tension induced by the nanosize curvature of critical nuclei could drive the metastable phase region of c-BN nucleation into stable phase region in the phase diagram. Consequently, c-BN nucleation would be prior to hexagonal boron nitride (h-BN) nucleation in competing growth of c-BN and h-BN upon HPHT supercritical fluid system.