A quasidegenerate perturbation theory based on multiconfigurational self-consistent field (MCSCF) reference functions is derived. The perturbation theory derived here is for multistate, where several MCSCF functions obtained by the state-averaged MCSCF method are used as the reference and an effective Hamiltonian is constructed by perturbation calculation. The energies of states interested in are obtained simultaneously by diagonalization of the effective Hamiltonian. An explicit formula of the effective Hamiltonian through second order is derived as well as general formalism, and is applied to calculate potential curves of the system H2, Be–H2, CO, NO, BN, and LiF. The results agree well with those of full configuration interaction or multireference single and double excitation configuraiton interaction methods for both the ground and the excited states.