The resistivity of different polymer composites is investigated in different temperature-changing processes, which demonstrates more complicated changing behaviors than that usually reported in literatures. The complicated resistivity-temperature (R-T) behaviors cannot be explained by current theories, and the contradictions of them with current theories are presented in this article. With the experiments, a new viewpoint on the R-T behaviors of polymer composites is proposed, i.e., the resistivity changes are attributed to internal stress generated when the morphology and structure of the composites undergo great changes. Based on this speculation, the complicated R-T behaviors of polymer composites can be well explained.