Thermoplastic polyolefin (TPOs) are widely used in automobile industry applications such as bumper and instrumental panels. The mechanical properties of polymer and polymer composites are temperature and strain rate dependent. Under cyclic loading, the inelastic deformation energy is dissipated as heat within the material, and consequently the temperature will rise, resulting in material failure. It is found out in this study that the amount of temperature increase is largely determined by the test frequency and load level. However, the test frequency is set as constant in the current ASTM standard. This could result in material failure due to overheating instead of fatigue failure under higher load level. In this paper, the material failure mechanism under cyclic loadings is investigated. A critical frequency-load map is proposed to identify the failure modes based on load levels and test frequencies. In order to reduce experimental effort, a modified viscoplasticity model is proposed to calculate the energy dissipate and temperature increase during fatigue testing to predict such critical frequency-load map for polymer materials.