Long Fiber Reinforced Thermo Plastics (LFRTP) is a composite material with high specific strength　and light weight. An injection molding of LFRTP is a well-established method in the automotive　industry to produce high quality structural parts in mass production. In the injection molding　plasticization of FRTP, fibers tend to be broken, and causing a negative effect on the mechanical　properties of molded parts. One of the solutions is to lower the shear stress in the plasticizing process　to prevent the fiber breakage. However, the reduction of shear stress decreases the mixing　performance of the screw, and low mixing performance tends to leave fiber agglomerates. Reducing　the fiber breakage and improving the fiber dispersion are an adverse relationship, and difficult to　improve both properties at the same time. The purpose of this paper is to suggest the effective screw　geometry for improving both of fiber dispersion and residual fiber length. Five kinds of screws were evaluated by investigating the molded specimen. It was found that screw geometry at the　compression zone of the screw has a dominate effect of the residual fiber length. Choosing proper　screw geometry for melting process and mixing process is the most important concept for improving　both of fiber dispersion and residual fiber length. Flow analysis was performed to interpret the　experimental results. Furthermore, we discussed the optimization of screw geometry for the plasticizing of GFRTP.