The perfection, reorganization, and isotropization of the mesophase of isotactic polypropylene were investigated by fast scanning calorimetry (FSC). The analysis was made at different heating rates, to evaluate the isotropization temperature of the initially grown mesophase and of mesophase which has been perfected or even reorganized into crystals. Mesophase formation is paralleled by formation of a rigid–amorphous fraction which was about twice the mesophase content. While the isotropization temperature of the initially grown mesophase is controlled by the temperature of its formation, the isotropization temperature of perfected or reorganized mesophase was independent on the condition of prior formation. Absence of exothermic heat flow during heating suggests that the mesophase converted gradually into crystals due to the fast heating rate. The discussion of data includes new ideas about nucleation, growth, and perfection of the mesophase, involving nanophase to microphase and rigid–amorphous fraction to mesophase transitions.