Various surface process methods have been developed to decorate plastic or metallic products. Film insert molding (FIM) is one of the methods that enhance the functional and/or aesthetic qualities of a product's surface. However, the drawbacks of FIM are that the thickness of the film can change, depending on the product configuration, and further, the pattern of the decorated film may change. Therefore, this article attempts to quantify the changes in the thickness and in the pattern of the decorated film during the FIM process. G'Sell's viscoelastic constitutive law was adopted to describe the rheological behavior of polymer film. A constant-velocity uniaxial tensile test at high temperature, which is a new method proposed in this research, was used to obtain the rheological parameters. We also suggested a visual method for predicting pattern change, which was validated by comparing analytical results with those of real products. POLYM. ENG. SCI., 49:2195-2203, 2009. (C) 2009 Society of Plastics Engineers