A method for predicting the behavior of the system including the simple hydrate phase under constant volume condition was presented. Governing equations for H-L_w-V coexistence under constant volume condition were derived based upon the mass balance, the apparent restriction about mole fraction (the sum of the mole fraction must be unity), equal fugacity criteria and the equation for the total volume of the system. Then, to prove the validity of the proposed method, experimental studies about the dissociation process of hydrates were performed. Temperature and pressure traces in the hydrate dissociation process were successfully predicted by the proposed method. The proposed method can predict the change of all intensive and extensive variables in the system with respect to temperature. Thus, it is very useful for engineering and research purposes.