Lightweight technology has been applied into many industries especially for automotive to enhance the fuel efficiency. One of most famous methods is applied fiber-reinforced thermoplastics (FRT) technology, it includes short and long fiber-reinforced thermoplastics (FRT) to support lightweight technology. However, the enhancement mechanism by the microstructures of the fibers in FRT is still too complicated to understand. In this study, we designed a benchmark to study the fiber microstructures based on ASTM D638 with dog-bond system. First, we have tried to study how the geometry of cavity influences the fiber orientation during the injection processes. Furthermore, we have paid the attention on the variation of the fiber length distribution as the injection molding processing. Results show that the geometry of cavity has significant effect on the fiber orientation during the injection processes. Since the system has contraction and expansion structure, the orientation tensor component a11 corresponding to the flow direction, will be enhanced and then decreased along the cavity. Moreover, the fiber lengths have dramatically sharp distribution on skin layer when melt goes through the gate into the cavity. It will allow almost 90% lengths are broken through the skin layer. Meanwhile, using numerical visualization from runner to cavity through core layer, there is about 30% length broken during the journey in runner section. Finally, some fiber orientation results are compared with some literature's. Results showed that our numerical predictions are matched with that of literature quite well in the trend.