In recent years, the lightweight technologies have become the driving force for people in automotives and others development. Among those technologies, using short and long fiber-reinforced thermoplastics (FRT) to replace some metal components can reduce the weight of an automotive significantly. However, the microstructures of fiber inside plastic matrix are too complicated to manage and control during the injection molding from screw, to runner, to gate, and to cavity, especially, the fiber length variation history during the screw plastification. In this study, we have integrated the screw plastification, to injection molding for fiber microstructures investigation. Firstly, the fiber breakage prediction during screw plastification has been conducted. Results show that fiber breakage is strongly dependent on screw design and operation. Specifically, before leaving the nozzle, the average length of long fibers is less than 25% of the original average length when they enter the hopper. When the screw geometry changed, even the compression ratio is lower, the fiber breakage could be higher. Furthermore, the broken fibers will be delivered into cavity to influence the quality of injected parts. The microstructure of concentration and orientation are investigated as well.