Polymer blending was used by adding different proportions (5, 10, 15 and 20 wt %) of the short carbon fiber into the high impact polypropylene (PP) matrix to reinforce the matrix’s mechanical property. The carbon fiber was melt blended with the PP matrix. The mixture was repeatedly processed by the single-screw extrusion into chips, which later became the composite plate by the injection molding machine. In this study, the effect of the repeatedly process on the PP/Carbon fiber composite plate’s mechanical property was examined. The tensile strength of the mixture having one process and six processes were 886 N and 857 N respectively. The impact strength of the composite plates processed for once was 334 J/m while that of the plates processed for six times was 325 J/m. The three-point bending of the composite plate processed once was 30 MPa when that of the plate processed for six times was 28 MPa. The more the times of the process, the weaker the composite plate’s mechanical property became. The carbon fiber was used to reinforce PP’s mechanical property. This study will be applied in the industry.