The impact of deformation on the demolding force is investigated both experimentally and by simulation for the thermal imprint process. The effects of substrate thickness are the focus of this article. In experiments, one mold is used throughout all experiments in order to exclude any effects of the mold fabrication process. The mold pattern is transferred to a polymethylmethacrylate (PMMA) film by the thermal imprint. The PMMA surface height is measured by a commercial height profiler after the imprint, and it agrees with lateral variations in residual layer thickness. Demolding force increases as substrate thickness increases. Substrate deformation is simulated by simplifying a model to assume the presence of large cavities. Substrate deformation in the simulation agrees with the PMMA surface height profile in the experiment. The normal force to the PMMA side wall by the mold pattern is important for estimating the demolding force. Since the normal force during the pressing process is simulated, the normal force is induced by both the lateral expansion of PMMA resin and substrate deformation. The normal force from substrate deformation is selected based on a detailed analysis of the simulated normal force. The obtained normal force agrees with the demolding force in the experiment. (C) 2010 American Vacuum Society. [DOI:10.1116/1.3517537]