In this work, an abnormal melting behavior of isotactic polypropylene (iPP) was investigated by polarized optical microscopy (POM), temperature-modulated differential scanning calorimetry (TMDSC), scanning electron microscopy (SEM), wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS). It can be found by POM that the melting of alpha-iPP spherulites isothermally crystallized at 135 degrees C could occur prior to that of a-iPP spherulites crystallized at 125 degrees C when heated by 2 degrees C min (-1), but the case is opposite if the heating rates are 10, 20 or 50 degrees C min -I. Meanwhile, the occurrence of lamellar thickening could be revealed by in-situ synchrotron SAXS, it can be revealed that the initial lamellar thickness of u-iPP spherulites grown at 135 degrees C is over that of spherulites grown at 125 degrees C, whereas the latter exceeds the former by means of lamellar thickening after heating. After a series of experiments, the lamellar thickening mechanism could be described as the following, i.e., the tangential lamellae in the spherulite melt and recrystallize on the radial lamellae during heating process, yielding much thicker radial lamellae with higher perfection. (c)2017 Elsevier Ltd. All rights reserved.