The effect of molecular weight of a compatibilizer on the mechanical properties of polypropylene/clay nanocomposites was investigated. Nanocomposites of polypropylene with organically modified clays were prepared by melt compounding of the three components, i.e., polypropylene, maleic anhydride-grafted polypropylene (PP-g-MA) as a compatibilizer, and organically modified clay. Three kinds of PP-g-MA with different molecular weight were used. The nanocomposites' clay dispersion and mechanical properties, such as flexural modulus and impact resistance, were investigated. X-Ray diffraction patterns showed that the maleic anhydride content of PP-g-MA affected the dispersion of clay particles. The mechanical testing results showed that the molecular weight of PP-g-MA profoundly affected the impact strength of the polypropylene/ clay nanocomposites.