In this study, we examine the effects of heating, nucleation, cooling, and reheating on the thermal properties and structure of metallocene isotactic polypropylene (m-iPP) that had been prepared initially in a standard state containing nearly equal amounts of the crystallographic alpha and gamma phases. Heat treatment was achieved through partial melting and annealing by the heating of samples to self-nucleation temperatures (T-n's) that spanned and exceeded the entire range of melting of the standard state, from 122 to 160degreesC. The relative amounts of alpha and gamma crystals are determined from the area under the unique wide-angle X-ray reflections. The lower and upper endotherms are caused by the melting of gamma and alpha crystals, respectively. Four distinct regions of T-n were identified on the basis of the thermal and structural parameters of m-iPP. In region I, T-n is below the peak melting temperature of the gamma phase. Here, gamma crystals are annealed and alpha crystals are barely affected by T-n. In region II, T-n is above the peak of the lower endotherm but below the peak of the upper endotherm. gamma crystals melt, and alpha crystals anneal. In both regions I and II, the portion of the sample melted at T-n recrystallizes epitaxially with existing parent alpha lamellae as the substrates, and the amount of alpha always exceeds the amount of gamma. In region III, T-n is above the peak of the upper endotherm, and all gamma crystals and some or all alpha crystals are melted at T-n. The number of alpha-crystal nuclei steadily decreases as T-n increases, causing systematic depression of the crystallization and melting temperatures seen during cooling. Finally, in region IV, T-n exceeds the upper endotherm, and only small self-nuclei or heterogeneous nuclei remain. Recrystallization is now suppressed to lower temperatures. For regions III and IV, a crossover behavior in the relative amounts of alpha and gamma is observed during cooling from T-n. Because of the effective nucleating ability of alpha toward gamma, as the temperature drops, the amount of gamma increases and then exceeds the amount of alpha. With subsequent reheating, the reverse crossover occurs because of the lower melting point of gamma.