Sodium 2,2 ' -methylene-bis-(4,6-di-t-butylphenylene)phosphate(NA-11) is widely used as a nucleating agent in the processing of isotactic polypropylene(iPP). To reveal its act as the nucleating agent on iPP, the crystallization behavior of NA-11 on isotactic polypropylene was investigated. NA-11 was crystallized on the oriented, thin iPP film from the solution in dimethyformamide at various temperatures. iPP was also crystallized from the melt on NA-11 crystals grown from ethanol. The samples thus prepared were examined by electron microscopy. NA-11 epitaxially crystallized on the iPP substrate at high temperatures, performing the crystallographical relationship, [010](NA-11)//[001](ipp) and (001)(NA-11)//(010)(ipp), and iPP on the NA-11 crystals with the same orientational relationship.