Isotactic polypropylene (iPP) and iron oxide (Fe3O4) nanocomposites were mixed by masterbatch blending technique in a single screw extruder machine. The concentrations of Fe3O4 in the iPP/Fe3O4 nanocomposites were 0.5, 1, 2, and 5% by weight. The influence of Fe3O4 nanoparticles on the effectiveness of nucleation, morphology, mode of crystallization, and crystallinity of iPP were studied by differential scanning calorimetry (DSC) and polarized light microscopy (PLM). The introduction of Fe3O4 nanoparticles in the iPP matrix inhibited the formation of β crystals, and caused a shift in the melting point to higher values. The magnitude of the shift was up to 20–21°C which indicates that using the masterbatch technique leads to an enhancement of the dispersion process of the Fe3O4 nanoparticle and the formation of less agglomerates in the iPP/Fe3O4 nanocomposites. The percentage crystallinity, Xc, increased at the low cooling rates of 1 and 2°C/min. At higher cooling rates of 5, 10, and 20°C/min, the masterbatch technique produced nanocomposites of Xc with nonuniform trends. The overall crystallization rate enhancement for the iPP/Fe3O4 nanocomposites is attributed to the presence of Fe3O4 nanoparticles as a nucleating agent which have no significant effect on the growth rate of iPP crystals. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010