Thermal discoloration and degradation of poly(ethylene terephthalate) filrn were examined over a range of 270 to 310˚C, above the melting temperature, without the addition of catalysts and stabilizers. The degree of degradation was measured by colorimefery and IR spectroscopy, and the thermal stability was determined from intrinsic viscosify(IV) and carboxyl end group(CEG) titration. The coloration of PET film was thought to be caused by unsaturated rnaterials produced by thermal degradation undernitrogen and by acids resulting from oxidation in air. The kinetics of thermal degradation was investigated by measuring changes in intrinsic viscosities and the number of carboxyl end groups. The activation energies(Ea), calculated from the changes of initial viscosities, were 45.6kJ/rnol in air and 158.8kj/mol under nitrogen, respectively. The activation energy for the changes of the number of carboxyl end group in air was 33.2kJ/mol. However, the value of Ea could not be obtained under nitrogen because of little decrease in the number of carboxyl end group.