Abstract: This paper studies the sensitivity of eigenvalues to ply angles for composite plates. Based on the classicaltheory of the laminated composite plate , the mass mat rix and stiffness mat rix were obtained by using thefinite-element-method and principle of virtual work. The equation between ply angles and stiffness mat rix was determined and by using the perturbation method in dynamic equation , the sensitivities of the eigenvalues to the ply angles were obtained explicitly. Through the numerical example given in the paper , the conclusion was demonst ratedthat the present method has superiority in accuracy compared with the finite difference method.