Glass-fiber reinforced polypropylene (GFPP) is used for many automotive parts. For use in a wider range of parts, this requires a higher strength.However, as PP is a non-polar polymer, the interfacial shear strength between PP and GF is weak. In order to resolve this problem, maleic acid grafted PP (MAPP) is used. In this way the tensile strength of GFPP can be improved remarkably. When the content of GF in GFPP is 30 wt%, the appropriate amount of MAPP is 2 wt%. Further, in recent vent injection molding machines, GFPP can be obtained by directly introducing fibers from a vent hole. It seems that the appropriate amount of MAPP differs between this fiber direct injection injection molding (DFFIM) and the short fiber GFPP compounded. In this study, the effect of the amount of added MAPP on the mechanical properties of GFPP prepared by DFFIM process and ordinary process is investigated.