Abstract: An approach, defined as the mode perturbation metho4 has been developed for modal analysis of the elastic moderate-thick plates. The solution of moan characteristics of the moderate thick p1ate is obtained approximately in subspace based on the lowe modal functions of the thin plate which has same numerical parameters and boundary condition with the moderate-thick plate except the thickness. By this method the complicated problem of differential equation of mohon is turned into the simple problem of algebraic eopations which is easy to solve. As to rectangular simplysupported moderate-thick plates, very simplified resuls are give in this paper Several numerical examples show that this method is praticable and has good preciaion. Finally, the shape effects of shear deformation and rotary inertia as well as the shape factor of the cross section on the natural frequencies of the plates are discussed.