Abstract: Short carbon fibers(SCF), wood flour(WF), high density polyethylene(HDPE)plastic, and other additives were mixed and compounded, then they were hot-press molded to SCF/WF/HDPE composite board. The effects of the carbon fiber content and surface sizing agent on the mechanical properties of the hybrid composites were studied. Carbon fibers were characterized by SEM and FTIR. The results showed that the tensile strength and bending strength of SCF/WF/HDPE increased by 8.4% and 10.6%, respectively, comparing with the control sample without carbon fiber when SCF content was 10%. When SCF content was 6%, the elongation at break and impact strength of SCF/WF/HDPE composite increased by 25.9% and 24.4%, respectively. The sizing agent on the surface of carbon fiber was benefit for bonding with WF/HDPE composite. When carbon fiber was used in the form of cloth, it could present much better reinforcement than that of short carbon fiber. Comparing with SCF/WF/HDPE composite, the tensile strength, the elongation at break, the bending strength and the impact strength were increased by 62%, 148%, 71%, and 313%, respectively, when carbon fiber cloth was attached to the surfaces of WF/HDPE composite board.