Organically modified Na-montmorillonite (MMT) samples were prepared by co-intercalation of octadecylamine (ODA) and stearic acid (STA) using a low-temperature melting procedure A blend of MMT and ODA (with the ratio 1:0.5) was stirred for one minute at 80degreesC in a laboratory mixer and then different amount of STA (weight ratios of MMT + ODA: STA = 1:0.3 and 1:1) were added and stirred for another five minutes. The resulting mixture was annealed at the same temperature for one hour X-ray diffraction (XRD) powder patterns of two co-intercalated samples contain a non-regular set of d spacings of basal diffractions. The first basal diffraction with d = 4.8 nm probably gives evidence for prevalent bilayer molecular arrangement in the interlayer and the non-regular set of d spacings of basal diffractions indicates possible mixed-layer interstratification and/or disorder in the bilayer arrangement of the interlayer space. The set of basal diffractions in the XRD patterns of co-intercalated samples significantly differs from the set of basal diffractions in the patterns of the samples intercalated by only ODA or STA. XRD patterns of Na-montmorillonite sample intercalated with ODA (in the ratio of MMT: ODA = 1:0.5) and with STA (in the ratio of MMT: STA = 1:1) contain broadening peaks. Polypropylene (PP) nanocomposite samples were prepared by mixing MMT modified in the described mode into the PP matrix. To improve the PP affinity to intercalated MMT maleic anhydride modified PP as a compatibilizer was used. The level of PP nanocomposites exfoliation was evaluated by XRD technique. The influence of the nanofiller on mechanical properties was measured by tensile tests, and an improvement of mechanical properties was noticed.