For poly(ethylene terephthalate), two equilibrium melting temperatures, 262 degrees C and 276 degrees C, were found with DSC. They should be assigned to two forms of crystals with the infinite crystal length, respectively. This finding made it possible every crystal form to derive the end surface free energy per unit area of crystals and the transition enthalpy of the ordered parts in the amorphous regions and using these thermodynamic quantities to convert DSC melting curves into the crystal length distribution. Through the derivation of them, the participation of the crystallization temperature on cooling in the thermal analysis of DSC melting curves was revealed thermodynamically. (c) 2008 Elsevier Ltd. All rights reserved.