With the rapid increase in the applications of polymer microreplication, it has become important to understand the replication process. Visualization is clearly the most direct and definite method of determining the replication process. In our experiment, the behavior of melt flowing over a stamper with V-grooves was observed through a prism glass installed in the mold; an ultrahigh-speed video camera connected with a long-distance microscope was used. As a result, when molding with an open cavity having V-grooves with a pitch of 100 μm and at an injection rate of 50 cm3/s, the filling of the melt into the V-grooves was generally completed during the first several milliseconds after the melt flowed over the grooves. However, as the pitch of the V-grooves decreased to 50 μm, the filling process slowed, thereby decreasing the filling ratio. In all cases, the filling of the melt into the V-grooves was accelerated when the injection rate increased, and this was followed by an increase in the filling ratio. In addition, the effects of mold temperature and cavity thickness on the filling behavior were also investigated. This visualization analysis offers a unique opportunity to understand and improve the replication process by injection molding. POLYM. ENG. SCI., 46:1590–1597, 2006. © 2006 Society of Plastics Engineers.