Viscous polymer flow in embossing stage during thermal nanoimprint lithography was investigated. Flow behavior of the thermoplastic polymer was analyzed numerically and experimentally. Fixed grid scheme along with the finite element method was used for numerical analysis. Effects of process parameters such as the temperature, the stamp speed, and the stamp geometry on the filling pattern for single cavity were studied. Also the patterns with several cavities were analyzed to understand the flow behavior between adjacent cavities. Experiments were performed using PMMA sheets to confirm the effects of parameters obtained from numerical analyses. Two different types of geometries were considered and the filled patterns were observed using a scanning electron microscopy. Numerical results exhibited a similar tendency as the experimental observation. POLYM. ENG. SCI., 51:209-217, 2011. (C) 2010 Society of Plastics Engineers