We investigated the effect of silica nanoparticles on the mechanical property and fracture toughness of two epoxy systems cured by Jeffamine D230 (denoted J230) and 4,4'-diaminodiphenyl sulfone (denoted DDS), respectively. Toughening mechanisms were identified by a tailor-loaded compact tension method which quantitatively recorded the deformation of a damage zone in the vicinity of a sub-critically propagated sharp crack tip. 20 wt% silica nanoparticles' fraction provided 40% improvement in Young's modulus for both systems; it improved the toughness of J230-cured epoxy from 0.73 to 1.68 MPa m (1/2), and for the other system improved from 0.51 to 0.82 MPa m(1/2). The nanoparticles not only stiffen, strengthen and toughen epoxy, but reduce the effect of flaws on mechanical performance as well. In both systems, nanosilica particle deformation, internal cavitation and interface debonding were not found, different to previous reports. This could be due to the various hardeners used or different identification techniques employed. The toughening mechanisms of the J230-cured nanocomposite were attributed to the formation and development of a thin dilatation zone and nanovoids, both of which were induced, constrained and thwarted by the stress fields of the silica nanoparticles. Regarding 10 wt% silica-toughen epoxy cured by J230, a thicker and shorter dilatation zone was found, where neither nanoparticles nor nanovoids were observed. With regard to the DDS-cured system, much less dilatation and voids were found due to the hardener used, leading to moderately improved toughness. (c) 2008 Elsevier Ltd. All rights reserved.