Phonon dispersions and vibrational density of states of MgSiO3 postperovskite, the new high-pressure phase of MgSiO3 perovskite, are calculated as a function of pressure up to 180 GPa using density-functional perturbation theory. The calculated frequencies are then used to determine the thermal contribution to the Helmholtz free energy within the quasi-harmonic approximation. The equation of state and several thermodynamic properties of interest are derived and compared with those of perovskite. The overall thermodynamic properties of postperovskite are almost the same as those of perovskite under its stability conditions.