Additive manufacturing (AM) technologies have been successfully applied in various applications. Fused deposition modeling (FDM), one of popular AM techniques, is most widely used method for manufacturing of plastic materials. Due to the poor strength properties of pure plastic materials, there is a critical need to improve mechanical properties for FDMfabricated pure plastic parts. One of the possible methods is adding reinforced materials (such as carbon fibers) into plastic materials to form carbon fiber reinforced plastic (CFRP) composites. The investigation in this paper is going to test if the properties of CFRP composites part will be enhanced compared with pure plastic part made by FDM. There are three major steps in this paper including producing thermoplastic matrix CFRP composites filaments extruded after blending plastic pellets and carbon fiber powder, printing parts in FDM process, and conducting tensile test. Effects of carbon fiber content and length on the mechanical properties (tensile strength, Young's modulus, toughness, ductility, and yield strength) of specimens are investigated. This investigation will also provide guidance for future investigations of fabricating thermoset matrix CFRP composites by AM techniques.