This paper shows the feasibility on the application of E-glass fiber/epoxy(GFRP) composite materials to an automotive leaf spring. In order to keep much lighter weight by replacing the steel with the composite material, it is important to optimize the material parameters and design variables consisting of the structure. This paper focused on the effects of material compositions and its fiber orientations for estimating the static behaviors of leaf spring. First of all, basic material properties of GFRP composite were measured by five types of coupon specimens from ASTM standard test. The reverse implementation was also done to obtain the complete set of in-situ fiber and matrix properties from ply test results. Finally, the static spring rates was examined for the variation of thickness and material parameters such as fiber angles and resin contents of composite leaf spring.