Abstract: Based on the zig-zag deformation model and first order shear strain effect laminated plate theory, a nonlinear buckling finite-element method of the composite sandwich plates is proposed, which contains both interfacial delamination and matrix micro-cracks damages. Furthermore, the transverse shear properties of the faces and the in-plane stiffness of the core are also taken into account. Furthermore, a delamination model and a multi-scalar damage model have been established, respectively. Because of the material properties of the face being degraded with the matrix micro-cracks propagation, the incremental constitutive equations are derived, and a modified Newton-Raphson incremental-iterative method is employed during the numerical calculation procedure. According to the models and analysis method provided by the authors, a nonlinear finite-element code has been developed. Some numerical examples are given for investigation of the effect of sizes of delamination and matrix micro-cracks, ply angles of the faces, and load cases upon the buckling behavior of the damaged composite sandwich plates. The numerical method and conclusions would be useful for the engineers of sandwich composite structures.