Clay-loaded polypropylene (PP) nanocomposites were fabricated via melt-compounding of two molecular weight (Mw) PPs (140 and 410 kg/mol) and octadecylammine-treated clay (C18MMT), with the assistance of maleic anhydride-grafted PP(PP-MAH), respectively, at 170 ℃ and 190 ℃. At both melt-compounding temperatures, the low-Mw PP tends to easily diffuse into silicate layers, especially in the presence of the mobile PP-MAH, resulting in a marked increase in silicate layer spacing (above 58 Å), when compared to 27 Å in the high-Mw PP-based system. Due to relatively lower melt-viscosity of the low-Mw PP-based system, however, there existed quasi-stacked clay aggregates with a thickness of 60∼80 nm, while the high-Mw PP-based nanocomposites showed relatively homogeneous dispersion of clays. The different morphologies are mainly related to changes in the viscoelastic properties of PPs, dependent on the processing temperature and their Mws. The slight differences in nanocomposites induce discernible crystallization and mechanical behaviors. High-Mw PP-based nanocomposites containing 1∼3 wt% C18MMT showed improvement in both tensile strength and modulus, while maintaining the inherent ductility of pure PP.