Kinetics of the solid-solid II-I phase transition of isotactic polybutene-1 was investigated. The fraction 1411 of phase I as a function of time t(tr) during the phase transition was measured by X-ray diffraction at various temperatures T-tr, The Avrami indices n of the W-I-t(tr) plots are approximately unity for T-tr > 288 K. A bell-shaped temperature dependence of the transition rate V with the maximum transition rate at 285 K was obtained. The V-T-tr curve and the Avrami index n = 1 suggest that the rate-determining process is primary nucleation. The dependence of V on T-tr for T-tr < 283 K is described by the William-Landel-Ferry (WLF) equation, which shows that the glass transition affects the transition rate. The Avrami index decreases to n < 1 for T-tr < 283 K, indicating a broadened distribution of the transition rate caused by the spatial heterogeneity of the amorphous state at low temperatures near the glass transition. Those evidences at low temperature clearly suggest that the solid solid phase transition is influenced by the mobility of chain folding, tie chains and cilia in the amorphous between the stacks of lamellar crystals. (C) 2010 Elsevier Ltd. All rights reserved.