An apparatus developed for microwave dielectric measurements at high temperatures was used to determine the dielectric loss epsilon" at 4.0 GHz between 20 and 105 degrees C for biaxially stretched and unstretched poly(ethylene terephthalate) (PET) films that had crystallinities of 35 and 5% at room temperature, respectively. The epsilon" for the former increased slightly with increasing temperature, whereas that for the latter showed a gradual increase up to 50 degrees C followed by a sharp upswing with further increase in temperature. These temperature changes in epsilon" are attributable to the shift of the dielectric beta relaxation to higher frequency with increasing temperature. Time dependence studies on epsilon" at 4.0 GHz and fixed high temperatures for the unstretched PET film showed a significant decrease in epsilon" with time only above 75 degrees C. This phenomenon may be ascribed to the crystallization of PET in the amorphous region. The microwave method described in this paper is useful in that it allows dielectric behaviour at high temperature to be measured quickly with a sample in the contact-free state.