The thermodynamic stability of clathrate hydrates I and II encaging Xe and Ar has been investigated by examining temperature dependence of the dissociation pressure on the basis of the generalized van der Waals and Platteeuw theory developed by Tanaka and Kiyohara, where the coupling of the host lattice vibration with the guest is taken into account in calculating the free energy of formation of hydrates. The predicted dissociation pressures of Xe and Ar hydrates agree reasonably well with experiments in higher temperature range. An agreement for Ar clathrate hydrate at low temperature is poor. This is circumvented by the use of a quantum mechanical partition function for a harmonic oscillator in evaluating the free energy difference between ice and empty hydrate.