Abstract: In this paper, based upon the basic equations of three dimensional theory of elastodynamics, the governing differential equations of thick pla ie have been formulated. The dynamic response of stress and displacement of thick plate subjected to the transversed forced are obtained. It is shown that the vibrational characters of thick plate consist of three modes: thickness shear mode, symmetric mode and anti-symmetric mode. The characteristic equations of simply supported thick plate are derived and the comparison of the free vibration frequencics based on the classic theory, middle thickness plate theory and three dimensional elasticity theory are given.