Fillers, such as glass fiber or carbon fiber, have been widely used to enhance the properties of polymer matrix. Reinforced polymer composite is a useful structural material to replace metal or ceramic structures. Lightweight structural material is one of key issues to make energy efficient vehicle and reinforced polymer composite is one of strong candidates to achieve this purpose. In real application, it is not easy to make thin, large or complicate structures due to high viscosity of polymer composite, especially high filler content. Crystallization behavior also affects the interfacial property between filler and polymer matrix. High content of short and/or long glass fiber reinforced nylon-6,6 is investigated to replace the metal structure in vehicle which requires high flexural strength and modulus. High content of glass fiber results in low melt viscosity and sometimes it causes poor mechanical properties due to poor wettability between glass fiber and polymer matrix. The use of viscosity improver has been reported, but in general poor mechanical properties were obtained. Newly synthesized chemicals from laboratory and some commercial materials are used to improve the melt viscosity. Effect of viscosity improver, filler content and processing conditions on the melt viscosity and crystallization behavior of composite is investigated. Compatibility of chemicals and polymer, size distribution of glass fiber are also investigated. Rheometer is used to measure the melt viscosity and morphology of composite are analyzed using X-ray and scanning electron microscopy. Differential scanning calorimetry is used to analyze crystallization behavior of composite under isothermal and nonisothermal conditions. Mechanical properties are obtained using universal tensile tester.