Drawing-induced g. crystal-mesophase transition of isotactic polypropylene (iPP) was studied with in situ wide angle X-ray diffraction (WAXD) during the true stress-true strain tensile testing. The true stress-true strain and in situ WAXD measurements revealed that the critical stress for the crystal-mesophase transition has a nearly linear correlation with lamellar thickness irrespective of the sample processing method, which indicated that superstructures might have minor effect in this aspect. Accordingly, a tentative mechanism of phase transition is proposed based on the ideas of nucleation and dislocation theories. The fitting result shows that the free energy of the stretching-induced mesophase is about 2.5 J/cm(3) lower than that of a crystal at room temperature. The energy barrier for the formation of critical nucleus during the a crystal-mesophase transition is estimated as about 71 k(B)T, which is possible to be overcome by the thermal fluctuation. The critical size of the nucleus decreases with the lamellar thickness l(c) as well as the critical tensile stress for the crystal-mesophase transition, which is rather similar to the effect of supercooling. (C) 2009 Elsevier Ltd. All rights reserved.