This paper intends to analyze the influences of the interface and interface regions (see wording in Adamson, A. W., "Physical Chemistry of Surfaces," Wiley, New York, 1976), between two phases on the equilibrium course of the phases and to discuss the phase equilibrium theory by means of the theoretical method of Gibbs under the conditions of the interface layer model of the physical interface. In this paper, the theoretical discussion is first. The phase equilibrium conditions of Gibbs are analyzed and the view points of some researchers on the phase equilibrium conditions of Gibbs are introduced. The influences of the interface and the interface region on phase equilibrium conditions are discussed. And the interface layer model of the physical interface is inducted into the research concerning equilibria inside one phase and between two phases. The theoretical method of Gibbs is used. The obtained chemical equilibrium condition is that the chemical potentials of the interface regions in two phases are the same. The demonstration of the above theory is researched next. For this, we must first check and confirm the correctness of our conclusions by means of thermodynamics and phase analysis. Then the data of some organic and inorganic compounds and elements are calculated; these practical data are used to prove the correctness of our theoretical conclusions. On this basis, it can be considered that both our theory and Gibbs' theory can be used in the domain of phase equilibrium theory. The difference between the two theories is that if Gibbs's theory is used, it must be by the aid of activity theory to preserve equilibrium between theory and practice. However, if our theory is used, this activity theory is not needed; only some thermodynamic parameters of experimental measurements are needed for the calculation.