Based on the carbon thermodynamic equilibrium phase diagram, a nanoscaled thermodynamic approach, taking the effect of nanosize-induced additional pressure on Gibbs free energy of diamond critical nuclei and phase transition probability from graphite to diamond into account, was performed to elucidate diamond nucleation on nondiamond surfaces upon chemical vapor deposition (CVD). The theoretical analysis showed that the capillary effect of the nanosize curvature of diamond critical nuclei could drive the metastable phase region of nucleation of CVD diamond into a new stable phase region in the carbon diagram. Consequently, diamond nucleation is preferable to graphite phase formation in competing growth of diamond and graphite upon CVD.