In recent years, using short and long fiber-reinforced thermoplastics (LFRT) to replace some metal components has become one of the most attractive lightweight technologies. However, the microstructures of fiber inside plastic matrix are too complicated to manage and control during the injection molding through the screw, the runner, the gate, and then into the cavity. On the other hand, to enhance the plastification for better melt quality, the barrier type screw is very popular. But how the barrier design will influence the long fiber breakage is still not clear yet. In this study, we have integrated the screw plastification, to injection molding for fiber microstructures investigation. Specifically, we have focused on fiber breakage history through a barrier structure screw in injection molding. Results show that when 13 mm carbon fibers go through the barrier screw, the length of fibers is down to 1.75 mm (the number average) at the exit of screw. It is 10% shorter than that when fibers go through the regular type screw system. Moreover, the fiber breakage phenomena for two types of fibers (glass fiber and carbon fiber) are also investigated. At the exit of the barrier screw, the number average of glass fibers is 2.21 mm. Clearly, the carbon fiber is easier to be broken. Furthermore, the carbon fiber length distribution has higher peak at the exit of the screw. This discovery is consistent with the experimental results of some current literature.