Abstract: In this work, the thermal annealing effect of nuclear track in ³²S ion irradiated PET film was studied by measuring the track etch rates. PET film of 25 μm in thickness were irradiated in a vacuum chamber with 113.7 MeV ³²S ions uniformly at room temperature. The heavy ions penetrated the film and the tracks were produced perpendicular to the film surface. The irradiated films were treated at temperatures ranged from 70 to 180 ºC for 1 s to 12 s by contacting with a thermostatically controlled heater. The heat treated films was chemically etched in a conductometric cell, its breakthrough times were determined from the conducting current curves, and then the track etch rates were derived. The experimental track etch rates shows the thermal annealing effect of ³²S ion tracks in PET film clearly. When the heat treatment temperature is higher than the glass transition temperature of PET film, the track etching rate of treated sample is smaller than that of untreated sample. And the track etching rate of treated sample decreases obviously with the increase of heat treatment temperature. Moreover, the track etching rate of the sample decreases with the duration of the heat treatment time. So the nuclear track thermal annealing effect increases with the increase of heat treatment temperature, and it increases with the increase of the heat treatment duration also.