Fused deposition modeling is a fast growing additive manufacturing technology with advantages of low cost, low operation temperature, consistent prototype accuracy, and flexible material change. Fiber-reinforced plastic composite parts have been developed by the fused deposition modeling process to improve the mechanical properties of fused deposition modeling-fabricated pure thermoplastic parts which cannot be used as load-bearing parts in the actual applications. However, porosity found at the fracture interfaces of fused deposition modeling-fabricated fiber-reinforced plastic composite parts may limit their potential for direct replacement in the functional applications. The problem may result from the larger length–diameter ratio of the chopped thin fibers used in fiber-reinforced plastic composite parts fabricating. One of the possible methods to reduce the porosity is employing small length–diameter ratio particles/flakes (such as graphite) instead of chopped thin fibers as reinforcements. However, mechanical performances of the graphite-reinforced thermoplastic composites are still unknown. In this article, comparisons on porosity and tensile properties between the specimens of carbon fiber-reinforced thermoplastic and graphite-reinforced thermoplastic fabricated by fused deposition modeling process were conducted in order to test the effects of reinforcements. Fracture interfaces of the specimens after tensile testing were observed.