Poly(propylene)/monoalkylimidazolium-modified montmorillontie (PP/IMMT) nanocomposites were prepared by in situ intercaltive polymerization of propylene with TiCl4/MgCl2/MMT catalyst. The PP synthesized possessed high isotacticity and molecular weight. Both wide-angle X-ray diffraction (XRD) and transmission electron microscopy (TEM) examinations evidenced the nanocomposite formation with exfoliated MMT homogeneously distributed in the PP matrix. A thermal stability study revealed that the nanocomposites possess good thermal stability.