This thesis is intended to investigate the feasibility of continuous extrusion processing of finecelled plastic/wood-fiber composite foams with a high wood-fiber content (50%). The main benefits of incorporating wood-fibers in plastics are the increased stiffness and lowered cost of the resulting composites. However, these improvements are usually accompanied by sacrifices in the ductility and impact resistance. These shortcomings can be removed by effectively foaming and incorporating a fine-celled structure in these composites. The moisture released from wood-fiber during processing is known to deteriorate the cell structure. CO₂, an environmental benign PBA, not limited by the decomposition temperature of CBA, was used as a blowing agent. A strategy of lowering the maximum processing temperature to suppress the moisture generation from wood-fiber was applied in this study. The lowest processing temperature and the optimal gas injection amount were identified, which successfully achieved the desired density 0.5-0.8 g/cm³ of the plastic/wood-fiber composite foam with a fine-cell structure.