Thermal analysis has been carried out on polyester (PET) fibers after subjecting them to different physical modifications, such as drawing and heat setting. The relationship between structure and the various thermal transitions observed in the thermograms of poly(ethylene terephthalate) has been examined. It has been shown that the endothermic transition near the glass transition region and the exothermic transition at about 140°C, observed for amorphous PET fibers, may be associated with mesomorphic phase changes. The premelting endotherm is sensitive to the orientation, crystallite size distribution, and thermal prehistory. This transition actually represents melting of smaller crystals and recrystallization into larger crystals. Heat of fusion does not always necessarily represent the actual crystallinity, or order of the fiber prior to differential scanning calorimetry and may be influenced by several factors. The fusion curves give more information regarding crystallite size distribution than crystallinity.