Abstract: The plastic limit loads of composite materials and structures are analyzed in both the macroscopic and microscopic scales. Based on the concept of a representative volume element reflecting the microstructures of the composites, the strength parameters of the composites can be calculated. By means of the homogenization technique of micromechanics method and the finite element method, the yield surface of a composite can be determined. Then an upper bound of the plastic limit load of a composite structure can be calculated by solving a nonlinear mathematical programming problem.