Abstract: For the binary phase diagram involving two phase layers, the layer curve can be individed into two parts l_α and l_β from the critical point. General relationships between the binary phase diagram and the thermodynamic properties of the phases have been given in this paper. It is shown that a knowledge of any four of the following six quantites is sufficient to permit the other two to be calculated exactly:the layers curve l_α and l_β, enthalpy of the a phase, entropy of the a phase, enthalpy of the β phase, entropy of the β phase. Consequently, some very useful formulas, such as the formulas for calculating activities from phase diagram can be dirven.