The isothermal crystallization kinetics of intercalatively polymerized polypropylene／montmorillonite (PP／MMT) nanocomposites and isotactic polypropylene (PP) was investigated by differential scanning calorimetry (DSC)．The relative crystallization degree of PP/MMT nanocomposites was decreased slightly after introducing montmorillonite．The isothermal crystallization process was described by the Avrami equation with exponent n≈2.4 for pure PP and n≈3.0 for PP／MMT nanocomposites．The half crystallization time (t1/2) of PP／MMT was much shorter than that of pure PP．The σe

interfacial free energy per unit area perpendicular to PP chains

of PP／MMT nanocomposites was calculated by Hoffman theory．The result showed that the σe was decreased with increasing the MMT content．