Abstract: An equivalent strength method for optimum design , of composite laminated plates subjected to an in-plane arbitrary loading is presented as an intuitive and simple engineering method. The method affirms that the design of composite laminate is optimum when the strength ratios of each lamina are epuivalent and equal to one. This is a criterion. It can be considered that the orientations of individual ply groups are determined, but the thickness of each group can be adjusted in accordance with the principle of the equivalent strength ratio, in result the failure of statical tension,compression or shear in each ply of laminate under the designed loads happens at the same time. In design optimization by the equivalent strength method the thickness and the weight of a laminated plate are minimized. Five numerical examples are given and compared with corresponding data in references[1]and[2]. The results show that the equivalent strength method is better than the methods employed in references[1]and[2].