Abstract: A simple theory for describing elastoplastic behavior(nonlinear stress-strainrelations) of composites has been developed in terms of Green’s function theory and per-turbation method. For the metal-matrix composites reinforced with ceramic particles, twodifferent solutions,namely average t- matrix approximation(ATA) and coherent-potential ap-proximation(CPA), have been given. A calculation exainple of SiCp /Al composites indicatesthat the theoretical predictions are consistent with experimental results。