Effect of composition and molecular orientation on the crystallization behavior of poly(ethylene terephthalate) (PET) was investigated using DSC with winding method. As compared with PET homopolymers, PET copolymers exhibited low glass transition temperature (Tg), but higher cold crystallization temperature (7, c) due to retardation of crystallization. POY fiber having partial orientation showed much lower 7, . than the corresponding polymer chip. When the POY differ was heated without tension, it showed a peak-shaped Is which is ascribed to the molecular relaxation. On the other hand, the fiber heated with tension by maintaining a constant length via a winding method during DSC measurement did not show relaxational Tg, and crystallized just after 79.