The thermal shrinkage of heat-set polyethylene terephthalate (PET) sheets at 85 degreesC after stretching at low speed and low temperature was similar to that for sheets stretched at high speed and high temperature if the crystallinities of these sheets were the same. However, differences between other physical properties, such as yield strength, were observed, which may be due to differences in the amorphous phase. The sheets stretched at the higher temperature and higher speeds may have different structures in the molecular segments in the amorphous phase, more relaxed than in the sheets prepared at the lower speed and temperature.