The isothermal and nonisothermal crystallization and the subsequent melting behaviors of the polypropylene (PP) component in the in-reactor alloy were studied systematically by a series of thermal analysis techniques. The alloy sample used in the present study is a newly invented in situ blend of polypropylene with high ethylene-propylene rubber content. The effect of annealing temperatures on the subsequent crystallization and melting process of PP was explained by the different molecular behavior in the three "temperature domains." Multiple melting endotherms were also observed for the nonisothermally crystallized sample, and were attributed to the dynamic melting process which consists of three steps: melting, recrystallization, and remelting. The melting temperature of initial mesomorphic phase is found to be very close to the temperature of recrystallization or reorganization. To separate the recrystallization process from the overlapping processes, the difference spectra of fast and slow heated DSC endotherms were used, which is based on a straight forward but reasonable assumption. In addition, temperature modulated differential scanning calorimetry (TMDSC) has also been used to study the melting of PP component. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 121: 1372-1383, 2011