A wide set of crystallization isotherms and the subsequent melting behavior of isotactic polypropylene (i-PP) were investigated using differential scanning calorimetry and nanocalorimetry with a very high rate in the cooling step. The latter technique offers, indeed, the distinct possibility to perform isothermal crystallization experiments at any temperature in between the glass transition and melting, as the test temperature can be reached at a cooling rate of 1000 K/s, thus, preventing crystallization during the cooling step. Isothermal tests after such fast cooling were performed at intervals of 5 K within the temperature range -15 to 90 degrees C, and a local exothermal overheating was observed. In particular, for each isotherm, the observed peaks were fitted using the Kolmogorov-Johnson-Mehl-Avrami model. The plot of the crystallization kinetics constant as function of temperature gives clear evidence of two kinetic processes. The subsequent heating scan performed starting from -15 degrees C showed an exothermic event, between 0 and 30 degrees C, due to the mesophase cold crystallization, for isotherms at a temperature lower than 20 degrees C.