The continuity of orthogonally rib-stiffened homogeneous core material was described using δ function basing its deformation on the firstorder shear theory. Composite panels were considered as Kirchhoff thin plate, taking the in-plane displacements of the face panels and the transverse displacement of the whole structure as variables, and the kinetics equations of the orthogonally rib-stiffened composite sandwich plate were derived using Hamilton’s principle. The approximate solution of free vibration of the orthogonally rib-stiffened composite sandwich plate with simply supported boundary was obtained by using the series solution form. The natural frequency results were validated comparing to the FEM simulation. Considering the damping of the composite panels, the effects of some geometrical parameters of the sandwich plates including panel thickness, rib-stiffened web thickness, web height and web spacing on the nature frequency and structural lossfactor were discussed.The results show that there is an optimal value of the panel thickness making the structural lossfactor max; when the areal density is identical, altering web thickness or web spacing have no effect on the natural frequency of the sandwich plate.