The fracture toughness of an epoxy molding compound (EMC) has been enhanced over a wide temperature range by the addition of a very low volume fraction of silica nanoparticles to the EMC filled with micro-silica particles, which induces macroscopic crack deflection and plastic deformation in front of the crack tip. To evaluate the fracture toughness (G(IC)) of these materials, the single edge notched bending (SENB) test was performed for a wide range of temperatures (from ambient temperature to 230 degrees C). The fracture toughness of the nano-silica filled EMCs was found to be improved in this temperature range by as much as a factor of two. Investigation of the fracture surfaces revealed that the micro-silica particles are covered with deformed matrix materials, which implies that the silica nanoparticles induced the crack to move into the interface between the micro-silica particles. Fractography results suggest that the silica nanoparticles act as surface modifiers of the micro-silica particles, which results in crack deflection and plastic deformation. (c) 2006 Springer Science + Business Media, Inc.