By using the in-situ reflection- absorption infrared technique, the glass transition temperatures of poly(ethylene terephthalate) thin films were determined by detecting the abrupt change in the absorbance ratio of two comparable bands at 1340 cm(-1) (trans conformation) and 1370 cm(-1) (gauche conformation) during heating. It was found that the T-g of thin PET films reduces with respect to that of bulk sample and has a great dependence on the film thickness. The conformational energy difference (DeltaE) between trans and gauche states of each film was also calculated. The results indicate that the thickness dependence of calculated AE is nearly consistent with that of T-g measured. Furthermore, the relaxation behavior of the PET chains in thin films prepared by spin-coating and the influence of annealing on the T-g measured were also investigated.