A numerical simulation of the hot embossing process with nonisothermal embossing conditions was carried out to observe the flow pattern of poly (methyl methacrylate) into microcavities. The microcavity was isomorphically downsized. The ratio of the cavity width over the cavity thickness was maintained constant at 8:1 throughout the analysis, while the cavity thickness varied from 200 μ m to 0.5 μ m. It was found that as the microcavity was downsized, the filling mechanism varied. For larger cavity thicknesses (e.g., 100 μ m), the polymer flow climbed along the wall of the heated die and was then compressed downward and squeezed outward. In contrast, for a smaller cavity thickness (e.g., 5 μ m), the flow was uniform and the wall-climbing flow was absent. This size effect was correlated with the uniformity (UNF) of the temperature distribution of the polymer substrate during the embossing process. For larger cavity thicknesses, the high temperature zone was localized in the vicinity of the die wall, and consequently localized wall-climbing flow occurred. The size effect in nonisothermal embossing was also studied ex perimentally, and localized flow was observed for larger cavities but not for smaller cavities. © 2005 Society of Plastics Engineers.