Amorphous poly(ethylene terephthalate) fibers in which the skin was removed were studied to expressly study the effect of amorphous molecular orientation on crystallization behavior. Thermal analysis was carried out on fibers with a wide range of molecular orientation using differential scanning calorimetry (DSC) under constrained and unconstrained conditions. The thermal behavior was correlated with structural characteristics such as amorphous orientation determined using wide-angle X-ray diffraction. We show for the first time a quantitative inverse linear relationship between the degree of amorphous orientation and the cold crystallization temperatures and heat of crystallization. Crystallization begins at a critical amorphous orientation of 0.18, and extrapolation shows that even at modest amorphous orientation of 0.27, the cold crystallization can start spontaneously at T-g and with no change in free energy. (C) 2004 Elsevier Ltd. All rights reserved.