Since the environmental problems and new stricter regulations are forcing the industries to introduce more ecological materials for their products, biodegradable materials have attracted increasing attention. Among these materials, Polylactic acid (PLA) is remarkable for its modulus, strength, chemical resistance. However, PLA could not be used for automobile industries for its low heat resistance and impact strength. Therefore, in this study natural fiber was introduced as reinforcements in order to improve the properties of PLA. And for various experiments, Polypropylene (PP) was used as matrix resin instead of PLA. Especially for improving the properties of PLA composites, surface treatments, annealing, and adding rubber elements were performed. With surface treatments, we found that the mechanical properties of composite were improved. And with annealing treatment, we found the remarkable increase of heat resistance of PLA composite. For impact property we added reactive acrylic rubber, and we increased the impact strength by a large margin. In making test of automotive real product, we tried 3 times and acquired the well molded product as acceptable level. But in the reliability test of the product we found that PLA composite has some weak points. In the future, we will develop the long-term reliability of PLA composites and apply to the mass production of automotive interior parts.