The effect of type of compounding on the dispersion state and mechanical properties of polypropylene (PP)/ bentonite nanocomposites were investigated. Nanocomposites containing 1.5, 2.5 and 5% in weight of bentonite were compounded of three ways: 1) PP and raw bentonite were added together in the hopper of a twin-screw extruder, 2) PP and modified bentonite with stearic acid were added together in the hopper of a twin-screw extruder, 3) PP, modified bentonite with stearic acid and 2% weight of stearic acid (as interface modifier) were added together in the hopper of a twin-screw extruder. Stearic acid was used as both a new interface and as nanobentonite surface modifier. The tensile and impact properties of nanocomposites were evaluated. Wide-Angle X-Ray Diffraction (WAXD) was used to study the exfoliation-intercalation of the nanocomposites. Transmission Electron Microscopy (TEM) was also employed to inspect the dispersion state of the nanocomposites. The toughness of the nanocomposites with the type 3 of compounding was higher than the pure PP. Nanocomposites with the type 3 of compounding also showed the best dispersion state. Stearic acid gave appropriate results for improving the toughness of nanocomposites (impact resistance and elongation at break). This acid accomplished the function of interface modifier. Small quantities were required in this study, and this fatty acid is distinguished by its relatively low cost.