The effect of delamination on natural frequencies of simply supported
rectangular cross-ply and angle-ply laminates was investigated by the finite strip method. The effects of delamination length, delamination location,panel aspect ratio, elastic modulus ratio, and fiber orientation on the natural frequencies of the laminate were studied in detail. Results show that the effect of the delamination on the natural frequency is increased with the increase of delamination length. When the delamination is centered in high curvature region of the mode 1 shape, the effect of delamination on the natural frequency is in inverse proportion to the bending stiffness ratio of the delaminated portion to the undelaminated portion. For a finite plate, the natural frequencies depend not only on the delamination length and location,but also on the aspect ratio of the plate. For the angle-ply laminates, the natural frequencies are function of delamination length, delamination location, panel aspect ratio, and fiber orientation.