The most difficult problems to deal with multireference perturbation theory are the size extensivity,invariance with orbital unitary transformations and correct description of molecular dissociation behavior.A new multireference perturbation series is derived based on the Rayleigh-Schr(o)dinger perturbation theory.It is orbitally invariant.Its computational cost is comparable to the single reference M(o)ller-Plesset perturbation theory.It is demonstrated numerically that the present multireference second and third order energies are size extensive by two types of super-molecules composed of H2 and BH monomers.Spectroscopic constants of F2(X1∑+g),Cl2(X1∑+g),C2-(X2∑+g),B2(X3∑-g)and C2 +(X4∑-g)as well as the ground state energies of H2O,NH2 and CH2 at three bond lengths have been calculated with the present theory.The dissociation behaviors of CH4 and HF have also been investigated.Comparisons with other theoretical models as well as the experimental data have been carried out to show their relative performances.