Abstract: The annealing effect of thermal treatment on heavy ion track in polyethylene terephthalate (PET) film was studied by track etching method. Ion tracks perpendicular to the surface and penetrating the film were produced by ³²S ions with 113.7 MeV. The thermal treatments were carried out at 70-240 ℃ for 1-300 s. The films were chemically etched into track-etched membranes, and then the track-etched membranes were observed by microscope. The experimental results show that the thermal annealing effect of ion tracks in PET films is obvious. At the same thermal treatment interval, the annealing effect of ion tracks becomes larger and larger as the temperature of thermal treatment increases. And at the same thermal treatment temperature, the annealing effect increases with the thermal treatment interval.