Plastic composites are increasingly applied to automotive mechanical parts which demand high strength and high stiffness to meet cost saving and weight reduction. However, plastic component design and molding have often involved a process of trial and error.Recent progress in numerical analysis, including the use of the FEM (Finite Element Method), has made it possible to obtain quantitative data on part deformation and in-mold material flow and to optimize mold geometries.This paper first describes a developemental process of an injection molded accelerator pedal made from glass fiber reinforced thermoplastics (Polyamide: PA, Polypropylene: PP). This process consists of an integrated CAE (Computer Aided Engineering) system which can be applied both to component and mold optimized design. (Fig.1 shows) This paper also discusses the effects of ribbed structure (Type I ∼ III) and the number of molding gates on parts. Finally this paper discusses the characteristics of the parts which are influenced by the material variety, glass fiber content (30 to 50 wt% ), fiber length (short fibers and long fibers) and so on.As a result, these techniques have made it possible to reduce the plastic pedal weight to less than half that of steel and meet all the specifications for the equivalent steel production pedal assembly. Bench tests and vehicle running test have shown excellent durability, high stiffness and high strength.