The aim of this study was to examine the effect of the polypropylene based resin on the properties of organoclay-PP nanocomposites prepared by melt compounding using a twin screw extruder. The characterization of the obtained materials was performed by X-ray diffraction (XRD), transmission electron microscopy (TEM), thermogravimetric analysis (TGA), melt flow rate (MFR) and mechanical tests. The study has shown the effect of the polymer matrix molecular weight on the possibility of producing by melt compounding nanocomposites based on PP homopolymers or heterophasic PP-PE copolymers and an organically modified montmorillonite, in presence of maleic anhydride-modified polypropylene (PP-g-MAH). An increase of mechanical properties was achieved both for homopolymers and heterophasic copolymers. However the reinforcing effect of clay dispersed in heterophasic copolymers was not as high as found for the homopolymers. (c) 2005 Elsevier Ltd. All rights reserved.