In this study, the thermal degradation properties of polyethylene terephthalate film has been examined by the capacitance, Tan δ, thermography, FTIR, and SEM results at temperatures of 90~170℃ and frequencies of 0.3~3,000 kHz. It was found that the capacitance decreased with increasing thermal imaging temperature, probably caused by weakening of chemical bond with increasing temperature. Tan δ decreased upon increasing temperature from 90℃ to 170℃, probably due to the molecular motion of COOH radical or OH radical. The FT-IR measurement reveals that no structural change of the material occurs upon thermal radiation. The SEM measurement shows that the material is stabilized by thermal decomposition with increasing temperature; however, excessive thermal degradation obstructs the stabilization of the material.