Due to the high demand of smart green, the lightweight technologies have become the driving force for people in automotives and others development in recent years. 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. In this study, we have integrated the screw plastification, to injection molding for fiber microstructures investigation. More specifically, paid most of our attention on fiber breakage prediction during screw plastification. 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.