In recent years, fiber reinforced plastic (FRP) materials have become one of the main lightweight technologies and have been widely used in industry, especially in the automotive and aerospace industries. However, the reason why fibers can reinforce plastics is because of their microstructural properties, and among these microstructural properties, the fiber orientation is the most important factor to influence the system. However, the fibers’ behavior inside the FRP matrix is very complicated, and it is usually not easy to be observed. In addition, during the injection molding process, there may be some interactions between the melt flow and the fibers. These interactions could be further affected by the viscoelastic effect of the materials. In addition, how these complex interactions will further affect the process and products is not yet fully understood. Therefore, in this study, we have tried to use a geometric system with three ASTM D638 standard tensile bars to conduct the influence of the interaction between the flow-fiber coupling effect and viscoelastic effect on FRP. Both CAE simulation analysis and experimental observation were utilized as the study methods. The results show that with considering the flow-fiber coupling effect, it is found that with the coupling effect the flow melt front exhibits the convex-flat-concave flow phenomenon; but without considering the coupling effect, it shows the convex-flat-flat type. This significant difference is one of the evidences caused by the flow-fiber coupling effect. This result is consistent with that observation by the literature[17]. Furthermore, in order to understand how the flow-fiber coupling effect influence the fiber orientation in injection molding, there are four different group systems have been considered, namely: (1) Basic group: without considering flow-fiber coupling and viscoelastic effects; (2) Coupling effect group: considering the flow-fiber coupling effect only; (3) Viscoelastic effect group: considering the viscoelastic effect only; (4) Coupling plus viscoelastic effects group: considering both flow-fiber coupling and viscoelastic effects at the same time. The simulation results of injection molding processes based on those four groups have been further investigated. The results show that the simulation results of the Coupling effect group are closest to the experimental observation in fiber orientation distribution. Although the results of the Viscoelastic effect group are also consistent with that of experimental observation, but they are not as good as that of the Coupling group. While the simulation results of the both flow-fiber coupling and viscoelastic effects group are not good enough. Based on these results, the flow-fiber coupling effect is significantly verified. Moreover, to realize how flow-fiber coupling effect can further influence the quality of the injected parts, the dimensions of the injected parts have been measured. Specifically, the Model I and Model II (ASTM D638 specimens), each one has been divided into three regions, named near gate region (NGR), center region (CR), and end of filling region (EFR). For each region, the three directional side dimensions have been measured using five smaples each time. Then the average dimension for each side of each region has been recorded. Moreover, the details of the average fiber orientation distribution have been discovered by divided each region into five sub-regions. The correlation between the average fiber orientation distribution with considering the flow-fiber coupling effect and the three directional dimension variations of each region can be constructed. For example, in the NGR of the Model I, from upstream to downstream, the A11 fiber orientation tension is getting larger, the A22 is smaller. From the side dimension measurement of Model I, the dimension in the flow direction becomes larger, and the dimension in the cross-flow direction becomes smaller. The results show that the fiber orientation variation is consistent with that of the dimensional change for Model I at NGR. Similarly, the relation can also be observed in other regions in Model I, and regions in Model II. Based on these results, the flow-fiber coupling effect can be further validated.