The surface stability and equilibrium morphology of MoO3 were investigated by the first principles calculations. The thermodynamic energies of the stoichiometric surfaces of MoO3 are in the order (0 1 0) < (1 01) < (0 0 1) < (1 00). It is found that the (0 1 0) surface has a lowest surface energy, which agrees well with the previous calculations. The energies of the non-stoichiometric surfaces were evaluated as functions of temperature and oxygen partial pressure. The results show that the energies of Mo-terminated surfaces decrease with temperatures, and increase with oxygen partial pressures, while the energies of O-terminated surfaces show the opposite rule. The equilibrium morphology of MoO3 was predicted by using the Gibbs-Wulff model, and then was compared with the other's experiments and theoretical results.