In this study, the three-dimensional state equation for the jth ply of a laminated thick orthotropic plate is established in the local coordinate system according to the knowledge which has been introduced in the paper by Sundara Raja Iyengar and Pandya (1983, Fiber Sci. Technol.18, 19–36). Because all the physical quantities appearing in the state equation are just the compatible quantities of the interfaces, it is extremly convenient to develop the state equation of the whole plate. Furthermore, the number of unknowns included in the final equations has no relationship with that of the plies of the plate. Exact solutions are presented for the statics and dynamics of a three-ply orthotropic thick plates with simply supported edges. Numerical results are obtained and compared with those of Srinivas and Rao (1970, Int. J. Solids Structures6, 1463–1481) and thin plate theory.