In this work, montmorillonite (Mt) was modified by octadecyl amine ethoxylate ether (A) which has two poly(ethylene oxide) segments. Montmorillonite/polypropylene (Mt/PP) nanocomposites were prepared by a melt blending technique using different contents of modified Mt with and without maleic anhydride grafted polypropylene (PPg). The intercalated and exfoliated structures of Mt/PP nanocomposites were characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Because of the typical intercalated and exfoliated structures, the mechanical properties of Mt/PP nanocomposites were improved significantly. The modified Mt in a PP matrix led to a significant reduction of melt viscosity and enhancement in Izod-notched impact strength and elongation at break. The Izod-notched impact strength and elongation at break of Mt/PP nanocomposites were 1.95 and 2.77 times as high as those of pure PP. The tensile strength of Mt/PP nanocomposites was also improved in the presence of PPg.