Improved mechanical properties of either clay or carbon nanotube (CNT)-reinforced polymer matrix nanocomposites are obtained by pre-treating nanoparticles and polymer pellets prior to a melt compounding process. The clay or CNTs are coated onto the surfaces of the polymer pellets by a milling process. The introduction of moisture into the mixture of the nanoparticles and the polymer pellets results in the nanoparticles more easily, firmly, and thoroughly coating onto the surfaces of the polymer pellets.