A long-fiber-reinforced thermoplastics (LFRTs) injection molding machine was designed and fabricated to have the capability to injection molding LFRTs and maintain fiber length during the molding process. Besides machine and cavity design, different molding process parameters would also affect the fiber length, fiber orientation, air traps and mechanical properties of LFRT parts. Thus, this paper is intended to study the influence of key processing parameters such as back pressure, screw speed, and melt temperature of injection molded LFRT parts on their fiber length, fiber orientation, air traps and mechanical properties. Image processing device such as optical microscope was used to identify the fiber length, fiber orientation and air traps. Mechanical test system was used to measure the tensile strength of the parts. Different melt paths were designed to count the influence of cavity design on the mechanical performance of the parts. The results showed that high screw speed and back pressure could damage and shorten the fiber length of the injection molded parts, and consequently cause fiber orientation varied under such process conditions. Higher melt temperature would decrease the viscosity of the polymer melt and increase fluidity thus better the mechanical performance of the parts. It was also found that parts at different mold cavity locations would exhibit different mechanical properties.