In recent years, the use of long fiber-reinforced thermoplastics (LFRT) has become one of the most popular materials in lightweight applications. However, the microstructure of the fiber inside the plastic matrix is quite complex, making it hard to fully control the fiber effect in the injection molding process. On the other hand, the screw structure and operation will affect the residual fiber length significantly. However, the mechanism of how the screw influences the fiber breakage is still not yet clear. Thus, in this study, we apply the screw plastication in the injection molding process to investigate the fiber microstructure mechanism. The results show that more severe fiber breakage occurs when the compression force increases abruptly. Generally speaking, the breakage increases from the compression to the metering section. One effective method to reduce the fiber breakage is to increase the channel depths in both the feeding and the metering sections while taking the plastic melt quality into consideration. Moreover, the fiber breakage phenomena for two types of fibers are also investigated. Evidently, carbon fiber is easier to be broken. Its length distribution has a higher peak at the exit of the screw. This discovery is consistent with the experimental results of the current literature studies. POLYM. ENG. SCI., 58:452-459, 2018. (c) 2017 Society of Plastics Engineers