The effect of particle size matching and mismatching on the processability, and the mechanical and physical properties of particulate reinforced composites is investigated in this study. These composites were made from dry powder-powder blends. Polymer and reinforcement materials were chosen, characterized and molded into composite plaques. For the same particle volume fraction (400/0), stiffness was found to increase, in general, as particle size decreased. The intimacy of mixing, stiffness and strength improvements depended upon the reinforcement type. These results were compared with predictions from simple micromechanics models to gain a better understanding of their physical behavior.