Abstract Drawing-induced α 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 α crystal at room temperature. The energy barrier for the formation of critical nucleus during the α 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.