The melting of crystals is a typical first-order phase transition. Pioneering experiments revealed that the melting of polymer crystals could be governed by the laws that were established based on the correlation between the melting temperature and the crystal size. Lamellar thickness is frequently employed to serve as a key structural parameter of polymer crystals that have a large surface area but are relatively thin. However, in the later stage of melting, the physical feature of thickening and melting 3D folded-chain nanocrystals that have a large surface area to volume ratio is still limited. In this study, the melting behavior of isotactic polypropylene, especially after the disappearance of lamellar stacks, was examined by in situ small/wide-angle X-ray scattering (SAXS/WAXS). When the annealing temperature (T-a) was <162 degrees C, a common thickening and melting process of lamellae could be observed. Upon continuously increasing Ta, the SAXS data showed a transformation from periodic lamellar stack structures to isolated nanocrystals; moreover, the transformation temperatures (TL-N) were found to change with the initial lamellar thickness. The SAXS patterns of nanocrystals that survived melting could be fitted by using a form factor for polydisperse cylinders. The fitting results indicated that the height of the nanocrystals increased with Ta from 36.9 +/- 2 to 43.6 +/- 2 nm. Interestingly, it was noted that thickening of nanocrystals was much more difficult than that of lamellae due to the relatively high activation energy for thermal perfection.