Maleic anhydride-modified polypropylene was compounded with commercially available surface-modified montmorillonite in a twin-screw extruder. Recompounding ensured the removal of visible tactoids from the extrudate but TEM and XRD techniques showed nonuniform dispersion of clay platelets. In this study, we investigated the mechanical and fracture properties of nanoclay-filled polypropylene. Emphasis was placed on the fracture characterization of the clay-filled polypropylene. Tensile strength and stiffness increased steadily with an increase in the clay loading. The toughness of compounded materials was characterized using rigorous fracture mechanics. J-integral fracture resistance decreased with an increase in the clay content. The resistance against stable crack growth was compared using the slopes derived from the J-R curve and the tearing modulus concept. A significant amount of crack growth resistance was evident in the nanoclay-filled polypropylene as opposed to other brittle nanocomposites such as the nylon-clay systems. (C) 2003 Wiley Periodicals, Inc.