In this contribution, we reported to utilize polystyrene-block-polybutadiene-block-polystyrene (PS-b-PB-b-PS), a commercial triblock copolymer to toughen epoxy thermosets. First, a PS-b-PB-b-PS triblock copolymer was chemically modified with hydroboration-oxidation reaction, with which the midblock was hydroxylated whereas the endblocks remained unaffected. It was found that the degree of hydroxylation was well controlled. One of the hydroxylated PS-b-PB-b-PS samples was then used as the macromolecular initiator to synthesize a poly(epsilon-caprolactone)-grafted PS-b-PB-b-PS via the ring-opening polymerization. It was found that the PS-b-PB-b-PS with poly(epsilon-caprolactone) grafts can be successfully employed to nanostructure epoxy thermosets; the "core-shell" microdomains composed of PB and PS were generated in the nanostructured thermosets. The nanostructured thermosets displayed improved fracture toughness. POLYM. ENG. SCI., 2019. (c) 2019 Society of Plastics Engineers