The isothermal crystallization kinetics of maleic anhydride grafted polypropylene (mPP) in hybrids with unmodified or organically modified montmorillonite (MMT), which have been prepared by melt intercalation in a mixer, has been investigated by using time-resolved wide-angle X-ray scattering (WAXS) and differential scanning calorimetry (DSC). WAXS and transmission electron microscopy (TEM) showed that the mPP/unmodified MMT hybrids formed macrophase-separated mixtures rather than intercalated nanocomposites, while mPP/organically layered silicate(OLS) hybrids formed exfoliated nanocomposites. The crystallization rate of mPP/ unmodified silicate hybrids became faster with the MMT addition because agglomerated silicates in the mPP matrix act as nucleating agents. On the other hand, the crystallization rate of the mPP/OLS hybrids was slowed with the OLS addition. This retardation of the mPP crystallization is probably due to the presence of well-dispersed exfoliated silicates as diffusion or displacement barriers for crystallization. Interestingly, these well-dispersed exfoliated silicates change the crystal orientation as well as the crystallization kinetics of mPP in mPP/OLS nanocomposites, showing larger (040) peak than (110) peak for the a form crystal in WAXS profiles.