Abstract: The derivation method of the thermodynamic equilibrium criterion is based on the basic conception of physical chemistry and the basic thermodynamic principle , and combined with the thermodynamic basic equation and the definition of the chemical potential. The wellOknown YoungOLaplace equation is obtained as a result of the derivation. A new derivation method of the
YoungOLaplace equation is also presented , based on the interfacial energy , the potential energy , and the total energy of the whole material system which should be minimum on the thermodynamic equilibrium condition. The phase equilibrium calculation model and application examples considered capillary pressure is showed at the end of this paper.

Key words: YoungOLaplace equation, thermodynamics, interfacial energy, potential energy, phase equilibrium