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In this study, effect of injection molding process parameters on fiber length and on mechanical properties of Polyamide-6 / glass fiber composite were investigated to produce higher performance composites. Polyamide-6 was first compounded with an E-grade glass fiber in a co-rotating intermeshing twin screw extruder. Then, by using this composite, twenty-five types of experiments were performed by injection molding by changing the barrel temperature, injection pressure, hold pressure, mold temperature, cooling time and screw speed. Izod notched impact, tensile, viscosity, heat deflection temperature, differential scanning calorimetry tests were performed on injection molded samples. By performing these tests, the effects of process parameters on mechanical properties and on fiber length were observed. In order to understand the variation in mechanical properties, thermal tests were also conducted. Also, fiber length distributions of the samples were measured.Experimental data show that fiber breakage decreases with increasing screw speed, injection pressure, however, fiber length increases with increasing barrel temperature, mold temperature and cooling time. Fiber length is almost not affected with the hold pressure. It is assumed in this study that crystallinity is not affected with injection pressure, hold pressure and screw speed. As barrel temperature and cooling time increase, crystallinity increases, however, as mold temperature increases, crystallinity decreases. Impact strength, tensile modulus and tensile strength increase, whereas elongation at break decreases with the average fiber length. Crystallinity affects the tensile strength and modulus positively. The tensile strength and modulus increase with increasing crystallinity.