Fiber reinforced composites are widely applied to construction, automotive and aerospace due to their high specific strength and light weight as compared to metal. However, traditional injection molding has been restricted to fabricate short fiber reinforced polymer composite. Direct fiber feeding injection molding (DFFIM) is developed for long fiber reinforced polymer composites and omitted the compounding process, which would shorten reinforced fibers. The reinforced fibers have been fed into vent barrel of DFFIM process. Processing parameters such as matrix feeding speed, fiber charging speed, injection speed and injection pressure as well as numbers of feeding fibers are controlled for desired fiber contents in the composites by DFFIM. Mechanical properties of the composites are mainly depended on the reinforcing fiber contents, the fiber distribution, the fiber dispersion and the fiber orientation. Generally, the properties of the composites increase with increasing contents of the reinforcing fibers. Nevertheless, in the previous research, tensile properties of the composites by DFFIM decreased when the fiber contents higher than 40 wt% at the controlled processing parameters. In this study, we focus on the presence or absence of carbon fiber (CF) sizing agent on the properties of polypropylene (PP) composite material. CF supplied by the company was desized with hot water above 90 ° C. PP reinforced with composites using these CF are manufactured in dumbbell specimens with 30 ton DFFIM machines. CF of 1200 Tex is supplied to the vent barrel of DFFIM. The matrix feed rate was set at 50 rpm and the fiber filling rate was set at 20 wt%. The effect of the sizing agent on the mechanical properties and morphology of PP / CF composites by DFFIM is characterized by tensile test, impact test and scanning electron microscope (SEM).