Crystallization of isotactic polypropylene was analyzed by fast scanning calorimetry (Flash DSC) at cooling rates between 1 and 4000 K s −1 . By quantitative analysis of the glass transition intensity and the specific transformation enthalpy, the amount of mobile amorphous (MAF), rigid amorphous (RAF) and crystalline fractions (CFs) and its dependency on the cooling rate were determined. During cooling, two different crystalline phases are formed. At slow cooling rates (below 90 K s −1 ), the α-phase is crystallized. At faster cooling rates mesophase aggregates are formed. The crystalline phase reduces the relaxation time in the MAF independently from the kind of the crystalline phase. The RAF is formed during the crystallization process. In the case of mesophase crystallization, the ratio between CF and RAF is independent of the crystallinity. For α-phase crystals, this ratio depends on the crystallinity. A model for the CF-RAF structure is derived from the reported results.