For producing wood plastic composites (WPCs) having lightweight and cost efficiency, a microcellular injection foaming process is applied in our present work. Two types of exothermic foaming agents (EFA) with different range of particle size are used for formation of microcellular structure in WPCs. This research examines effects of particle size and concentration of the exothermic foaming agents on mechanical and physical properties of WPCs. The average cell size in WPCs is decreased with increasing the content of exothermic foaming agents. The large foaming agent shows small cell size compared to the small foaming agent case. Cell population density of foamed WPCs increases with increasing the EFA content, but decreases with decreasing the size of EFA. Concerning density and void fraction of foamed WPCs, the density is reduced but the void fraction is increased with increasing the EFA concentration. For the flexural strength, increment of amount of EFA decreases the mechanical strength because of the increase of cell density and void fraction. Also, the smaller size foaming agent (an AC9000 case) increases more strength due to influence of EFA size.