Carbon fibers are used as reinforcement in composites because it has an excellent specific strength, modulus, low thermal expansion. In particular, carbon fiber-reinforced thermoplastic composites have been receiving attention in various industrial fields because it is relatively easy to manufacture and suitable for mass production. However, the short carbon fiber reinforced composites which are manufactured by extrusion, injection molding experience shear stress during the process. Fiber length and mechanical properties of the final composites are affected by the shear stress which is generated during the extrusion, injection molding process. In this study, Pitch-based carbon fibers which have each different modulus are used as reinforcements and it is manufactured to the composites by same extrusion, injection conditions. Fiber length, tensile strength and impact strength of the composites are measured to confirm the effects of shear stress to the reinforcing fibers. The result showed that the carbon fiber which has lowest fiber modulus has a most long fiber length in final composites and it has excellent mechanical properties than other fibers which have higher modulus. This phenomenon appeared more clearly when carbon fiber contents are higher. Keywords: Carbon fiber, Composites