Fatigue fracture behavior of slowly notched polyethylene terephathalate (PET) polymers were investigated at temperatures close to their beta transition temperatures up to well above their glass transition temperatures. Detailed characterization on the morphology of the notched roots showed that the crack tip during crack propagation became more dull with increasing testing temperature. The failure cycle (N(f)) of these samples increased with increasing temperatures until it reached the alpha transition temperatures of PET polymers, and most of the increase in N(f) is due to the increased time consumed in the initiation period. On the other hand, the initial crack growth rate increased significantly and N(f) of these samples decreased dramatically as the temperature increased well above the glass transition temperature. This interesting temperature dependence of fatigue behavior is explained due to the change of molecular motion of PET polymers at this temperature range.