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The mechanical properties of polypropylene-bentonite nanocomposites were studied in this work. In this study, stearic acid was used as both a new surface modifier of the nano bentonite and a new interface modifier during the compounding of the nanocomposites with a twin-screw extruder. Three different weight concentrations (1.5 wt.%, 2.5 wt.%, and 5.0 wt.%.) were chosen for each type of nanobentonite compounds. Fourier transform infrared spectroscopy allowed us the possibility to discard any chemical interaction between the nanobentonite and stearic acid. Nevertheless, the physical interactions between both components favored the mechanical properties, resulting in around 150% improvement in the elongation of the nanocomposites containing stearic acid as surface and interface modifier. This can be due to the good intercalation of the nanobentonite platelets as found by wide angle X-ray diffraction and this was further confirmed by scanning electron microscopy, where the fracture surface analyses of these nanocomposites showed the best dispersion and wetting of the nanoplatelets by the polymer matrix. Crystallization behavior was also modified by stearic acid incorporation and the nanocomposites with better dispersion exhibited crystallization temperatures similar to pure polypropylene. [ABSTRACT FROM AUTHOR]