This paper presents a multilevel approach to the optimal design of laminated composites with consideration of natural frequency constraints The Ply thicknesses and orientations are designed to minimize the volumes of plates subjected to constraints on the first natural frequency. The optimization process is carried out in a dual scheme. In the first optimization level the optimal ply orientations which give the maximum natural frequency are determined. In the second level the optimal ply thicknesses are determined so as to minimize the structural weight, while maintaining the ply angles obtained in the first level. The process is repeated several times until convergence is obtained. Weight minimization is performed by two different methods: the constrained gradient travel method and the optimality criterion method.