Based on Mori-Tanaka’s concept of average stress in the matrix and Eshelby’s equivalent inclusions theory, the stress or strain of the matrix, the reinforced particles and the composite are derived under a prescribed traction boundary conditions. The plastic strains and strains due to thermal mismatch between matrix and reinforced phase are considered as eigenstrains. The matrix and composite are postulated isotropic and the matrix satisfies isotropic hardening law. The interface debonding is decided by the tensile strength of the particles whose debonding probability is described by Weibull distribution function. Then the overall elastoplastic constitutive relation of spherical particle-reinforced metal matrix composite is derived by secant modulus method considering the interface debonding. The theoretical uniaxial stress-strain bebavior of the composite agrees well with the experimental curves.