Abstract: By using the FTIR technique, the glass transition temperatures and chain conformation of poly(ethylene terephthalate) (PET) thin films were determined by detecting the abrupt changes in the absorbances of characteristic absorption bands during heating (30～170℃). It was found that the absorbance intensity of gauche conformation reduced with respect to that of trans conformation along with the crystallization. Moreover, the contents of gauche and trans conformations connected with CH2 wagging vibrations were calculated at different temperatures before and after cold crystallization.