In this paper, natural frequencies of square laminated composite plates for different supports at edges are presented. Using a third order shear deformation theory of plates (TSDT), which is categorized in equivalent single layer theories (ESL), a new set of linear equations of motion for square multi-layered composite plates has been derived. Laminated plates are supposed to be either angle-ply or cross-ply. Moreover, FEM is used to solve the equations and find the fundamental natural frequencies. Finally some results for plates with different combination of layers and supports are reported. The results are compared to the results of other ESL.