In the present research, the thermal stability and fire properties of polypropylene (PP) have been improved through direct melt intercalation of PP, organically modified montmorillonite (OMMT), calcium carbonate (CaCO3) nanoparticles, and conventional flame retardants, i.e., decabromodiphenyl oxide (DB) and antimony trioxide (AO). The morphology of the compound was characterized by means of X-ray diffractometry and transmission electron microscopy. Thermogravimetry analysis (TGA), cone-calorimetry, limiting oxygen index, UL-94, and tensile tests were also employed to investigate thermal and mechanical properties as well as the flammability of the compounds' Data, obtained from TGA, indicated that simultaneous incorporation of both OMMT and CaCO3 nanoparticles forms a synergistic effect to improve both the thermal and thermo-oxidative stability. The kinetic analysis of polymer degradation showed that the presence of nanoparticles hindered the thermal degradation of PP. The combination of OMMT and CaCO3 was more effective to improve fire properties than OMMT and DB/AO. The experimental results indicated that the incorporation of OMMT and CaCO3 improved both the tensile (i.e., the increase of yield strength, tensile strength, and Young's modulus) and thermal properties. (C) 2008 Wile, Periodicals, Inc. J Appl Polym Sci 110: 2971-2979, 2008