Due to its excellent properties, the fiber-reinforced plastics (FRP) material has been applied into industry as one of the major lightweight technologies, especially for automotive or aerospace products. However, since the microstructures of fiber inside plastic matrix are very complex, they are not easy to be visualized and managed. Specifically, the connection from microstructures to the physical properties of the final product is far from our understanding. In this study, we have tried to correlate from fiber microstructure feature to macro-property for fiber reinforced thermoplastic (FRP) in injection molding process. Specifically, we have designed an injection molding system with three standard specimens based on ASTM D638 where those specimens have different gate designs. The different gate design can change flow behavior and further provide different fiber orientation distribution (FOD) features. In the presence of fibers, the warpage of the full model can be diminished significantly. In addition, the improvement of mechanical property by introducing fiber into PP, in the case study, either through the side gate or the sprue gate, the tensile strength of PP+SF is over three times than that of pure PP. However, it is noted that the strength of Model I (side gate design) is always greater than that of Model II (sprue gate design) experimentally in the same injection molded part. The reason why the tensile strength of Model I is greater than that of Model II, it is due to the design-induced fiber orientation to melt which will further enhance the tensile strength of Model I. Finally, a series simulation studies with FOD have been performed. Results of FOD difference are consistent with the observation of the tensile strength difference.