The Thermally Stimulated Current (TSC) spectra of a series of Sub-Tg annealed Polyethylene Terephthalate (PET) specimens have been measured. It is found that there is only one peak at 80℃ above room temperature
which relates to the thermo-relaxation of frozen-in dip oles. The activation energy of such dipole motion has been calculated. The relation between the maximum current and the storage time can be explained by the free volume theory and agrees with the results from the exess thermodynamic properties. Compared with differential scann-ing calorimeter (DSC)and tensile stress-strain method
TSC is asimpler nad more sensitive method in studying Sub-Tg annealed polymers.