The relationship between heterogeneous or homogeneous nucleation and self-nucleation of polypropylene (PP) and PP nucleated by an organic phosphate salt (PPA) was studied by DSC. For pure PP, it homogeneously nucleated during cooling after melting at the selected temperature (T-s) of 170-200 degreesC for 3 min, but at the T-s of 160-168 degreesC self-nucleation occured; PPA only nucleated heterogeneously at the T-s of 168-200 degreesC, and there existed self nucleation at the T-s of 160-168 degreesC. The double melting peaks of PP and PPA at the T-s of 162 degreesC were observed. Once the self-nucleation occurred, the change of the crystallization temperature and heat of fusion of PP is more significant than that of PPA with the change of the T-s, depending upon the crystallization conditions. Results were explained by homogeneous nucleation, heterogeneous nucleation, self-nucleation, and annealing crystallization.