The fiber-reinforced plastication processes during the injection molding of reinforced resins with long fibers are difficult to observe through visualization cylinders because of possible obstacles, such as (1) opacification of melts, (2) abrasion on the inner surface of glass windows, and (3) breakage of glass windows under high pressures. In this study, we visualized the plastication process of long-glass fiber-reinforced resins containing 50 wt% fibers. In plastication experiments using three full-flight screws with different compression ratios, we observed the instability of the melting process, the generation of solid beds and melt pools, and the breakup of melted segments. We demonstrated that the plastication process depends considerably on the compression ratio and clarified the characteristic molding phenomena related to fiber breakup. Based on our observations, we proposed a model that explains the different fiber breakage distributions at different compression ratios during the melting process of long-glass fiber-reinforced resins. POLYM. ENG. SCI., 59:1300-1309 2019. (c) 2019 Society of Plastics Engineers