The thermal shrinkage behavior of drawn polymers is a complex process particularly if the crystallization occurs during the drawing process. In the present work, a theory on the mechanism which could explain the thermal shrinkage behavior of semicrystalline PET was suggested. On the basis of this mechanism, the thermal shrinkage behavior could be divided into three regions based on the morphological changes upon drawing. Consequently, this theory could explain the relationship between the thermal shrinkage ratio and the strain-induced crystallinity caused by drawing at a temperature range above the glass transition temperature.