Modified polyphenylene oxide (MPPO) is a widely known thermoplastic polymer and is extensively utilized for weight reduction of automobiles since it possesses outstanding mechanical properties, such as resistance and toughness. In this study, the viscoplastic behaviors of MPPO with respect to changes in temperature were identified through an approximate optimization method. For this work, parameter studies were conducted with a tensile simulation of the MPPO polymer, in accordance with the modified two-layer viscoplastic material parameters and the suggested shift parameter. The sensitivity tendency of the force-displacement curves was captured in accordance with the material parameters. Computational experiments for obtaining calibration error data were performed using the material parameters based on interior central composite design. Surrogate models of root mean square error relative to force and displacement were generated using the response surface method. The accuracy of the surrogate models was evaluated with R-square. Optimization for obtaining the material parameters was performed using the non-dominant sorting genetic algorithm-II. The results showed that the calibration error from the finite element analysis was minimized and agreed with experimental data. POLYM. ENG. SCI., 59:E200-E211, 2019. (c) 2018 Society of Plastics Engineers