Isotactic polypropylene samples annealed at three temperatures were used to explore their fibrillation behaviors during tensile deformation at elevated temperatures via in situ synchrotron small and wide-angle X-ray scattering techniques. Fibrillation denotes the transition of the isotropic spherulitic morphology into a highly oriented one during tensile stretching of a semicrystalline polymer. It was found that the fibrillation was accomplished by a stress-induced melting and recrystallization process. Three regions, identified as fibrillation with the formation of only mesophase, fibrillation with the formation of both mesophase and alpha crystallites, and fibrillation with the formation of complete alpha modification, were identified in a map of deformation Amorphous and annealing temperature. Such results are tightly linked to the different molecular mobilities of the samples prepared at different annealing temperatures and deformed at different temperatures. Lower annealing temperature and higher deformation temperature facilitate formation of alpha modification after stress-induced mechanical melting of the original crystallites. However, thicker lamellar crystallite deformed at lower temperatures presenting limited chain mobility ends up with a large amount of oriented mesophase structure.