Poly(epsilon-caprolactone)-block-poly(butadiene-co-acrylonitrile)-block-poly(epsilon-caprolactone) triblock copolymer was synthesized via the ring-opening polymerization of epsilon-caprolactone with dihydroxyl-terminated butadiene-co-acrylonitrile random copolymer. The amphiphilic block copolymer was used to toughen epoxy thermosets via the formation of nanostructures. The morphology of the thermosets was investigated by microscopy, transmission electronic microscopy and small-angle X-ray scattering. It means of atomic force. was judged that the formation of the nanostructures in the thermosets follows the mechanism of reaction-induced microphase separation. The thermal and mechanical properties of the nanostructured thermosets were compared to those of the ternary blends composed of epoxy, poly(butadiene-co-acrylonitrile) and poly(epsilon-caprolactone) with the identical content of the modifiers. It is noted that at the same composition the nanostructured thermosets displayed higher glass transition temperatures (T(g)s) than the ternary blends, which was evidenced by dynamic mechanical analysis. The fracture toughness of the thermosets was evaluated in terms of the measurement of critical stress field intensity factor (K-1C). it is noted that at the identical composition the nanostructured blends significantly displayed higher fracture toughness than the ternary blends. In addition, the K-1C of the nanostructured thermosets attained the maximum with the content of the modifier less than their counterpart of ternary blending. (C) 2009 Elsevier Ltd. All rights reserved.