Abstract: The dimethyl formamide（DMF） treatment effect in PET irradiated with 142 MeV ³²S was investigated. It is found that the change of track etching rate with changing the time and temperature of DMF treatment, with changing the time dipping in water before etching and changing the time stored in air after DMF treatment. New phenomenon observed in polymer dipped in water for more than 10 min between DMF treatment and etching process is also described in the paper.