The tensile properties and the fracture toughness, based on the essential work of fracture (EWF) method, of melt-compounded polymer nanocomposites based on polypropylene (PP) with organo-modified clays (montmorillonite) and maleic anhydride (MA)-grafted PP coupling agents were studied. Depending on the compounding sequence and on whether a coupling agent was used, some improvements in tensile properties were observed. These improvements were related to the level of dispersion of clay particles. The highest tensile properties were obtained for the PP/clay compound showing the highest surface density of uniform sub-micron particles. The mechanical improvements of the PP/clay compounds were those of a microcomposite in which the fiber reinforcement has an average aspect ratio of 17-35. The PP/ clay compounds with coupling agents and with highest surface density of uniform sub-micron particles showed very good fracture toughness, with EWF values slightly higher than those of unfilled PP. Fractographic observations showed that clay particles acted as void nucleation sites, which then grew and coalesced, promoting fibrillation of the remaining material between the voids. The EWF results indicated that the void nucleation density determined the fracture toughness. The EWF results also indicated that the plastic work dissipation related to the stability of the fibrillation, which was promoted by the use of a coupling agent. (C) 2004 Society of Plastics Engineers.