Melt-crystallized isotactic polybutene-1 (it-PB1) has been investigated over a range of crystallization temperatures from 40 to 90 degrees C, using small angle X-ray scattering and density measurements. The lamellar crystal thickness demonstrates two linear dependences on inverse supercooling, with a transition from one dependence to the other at about 65 degrees C. Each of the two dependences is in accordance with the nucleation theory in the high and low ranges of supercooling. The chain folding free energy determined for the low supercooling range is larger than that determined for the high supercooling range. Possible mechanisms for the transition are discussed, taking account of the entropy of chain folding directions. (C) 2008 Elsevier B.V. All rights reserved.