The influence of an organic phosphate derivative in the crystallization of the monoclinic phase of isotactic polypropylene was studied by differential scanning calorimetry. To analyze the nucleation activity of the additive, the self-nucleation process of the pure polymer was also studied by thermal techniques. A large increase in crystallization temperatures was obtained even for the lowest concentration of the additive, and its nucleating efficiency is the highest observed for a-nucleating agents in isotactic polypropylene. The nucleating agent was also observed to increase the stability of the crystals formed.