The size effect on particles reinforced metal matrix composites (MMCp) was investigated by a numerical method. A numerical multi-particles unit cell model has been constructed to carry out numerical analysis. In this model, circle particles were randomly placed in matrix according to uniform distribution, the sizes of particles in the paper were classified into 4 groups: 6μm, 13.5 μm ,50μm, and 100μm respectively. For investigating the effect of particles’ size on the plastic behavior of MMCp, the Griffith fracture criterion and the damage of ceramic particles were considered. Result showed that there was a close relationship between the particle size and the deformation behavior of the composites. Yield strength and plastic work hardening rate of the composites increase with decreasing particle size. The predicted stress–strain behaviors of the composites were in agreement with the experimental results.