Abstract: Based on the secant modulus method and Weibull statistic law, a microm echanical method is advanced to model the matrix plastic deformation and the particulate cracking for metal matrix composites during straining. The influences of the particul at emean size and its scatter on the composite macroscopic behavior are analyzed in details. It is found that the particulate mean size plays a key role in the composite damage properties. The theor etical results fora SiC_P/Al composite agree well with the experimental ones in the literature.