In nano-imprint lithography (NIL) process, which has shown to be a good method to fabricate polymeric patterns, several kinds of pattern defects due to thermal effects during polymer flow and mold release operation have been reported. A typical defect in NIL process with high aspect ratio and low resist thickness pattern is a resist fracture during the mold release operation. It seems due to interfacial adhesion between polymer and mold. In the present investigation, FEM simulation of NIL process was carried out to predict the defects of the polymer pattern and to optimize the process by FEA. To achieve the above mentioned purpose, FEM simulation technique based on constitutive modeling of polymer with experiments was firstly investigated . Secondly, the embossing operation in NIL process was investigated in detail by FEM. From the analytical results, it was found that the non-uniform flow-pattern of polymer and the applied pressure in the embossing operation induce the cavity and the drastic lateral-strain at the edge of pattern. It was also shown that the low polymer-thickness result in the delamination of polymer from the substrate. It seems that the above phenomena cause the defects of the final polymer pattern.