The effect of isotope on the isothermal crystallization behavior has been investigated for a series of isotactic polypropylene blend samples between the hydrogenated and deuterated species by using the thermal analysis technique. The analysis based on the Avrami equation revealed the crystallization rate constant k and Avrami exponent n, which were found to change systematically with the D/H content at the same degree of supercooling, reflecting the cocrystallization phenomenon of these two species. The k value was appreciably higher for the D species than for the H species. Correspondingly, the spherulite size, observed by polarized optical microscope and small-angle light scattering, was found to be larger for the blend with higher H content.