The mechanical properties of UHMWPE (Ultra-High-Molecular-Weight-Polyethylene) fiber when subjected to heating at different temperatures in a given time (5 seconds) were discussed. The results showed that with the increase of temperature, the strength，breaking elongation, modulus and work of rupture of the fiber are all decreased to a certain extent. Especially the strength and work of rupture decreased sharply，but the breaking elongation only changed slightly. The optical microscope of QUESTER showed that the longitudinal fracture morphology of the fiber when treated at different temperatures were different. With the increasing of temperature, the microfibril which had caused by the weak force of macromolecule remained no more, but the fine cervical and microporous part appeared. This demestrated that the UHMWPE fiber is poor in heat resistance. The higher the temperature, the easier the fiber breaks.