In this work, we investigated the effect of formation mechanisms of nanophases on the morphologies and thermomechanical properties of the nanostructured thermosets containing block copolymers. Toward this end, the nanostructured thermosets involving epoxy and block copolymers were prepared via self-assembly and reaction-induced microphase separation approaches, respectively. Two structurally similar triblock copolymers, poly(e-caprolactone)-block-poly(butadiene-co-styrene)-block-poly(e-caprolactone) (PCL-b-PBS-b-PCL) and poly(epsilon-caprolactone)-block-poly(ethylene-co-ethylethylene-co-styrene)block-poly(e-caprolactone) (PCL-b-PEEES-b-PCL) were synthesized via the ring-opening polymerization of e-caprolactone (CL) with am-dihydroxyl-terminated poly(butadiene-co-styrene) (HO-PBS-OH) and a.,w-dihydroxyl-terminated poly(ethylene-co-ethylethylene-co-styrene) (i.e., HO-PEEES-OH) as the macromolecular initiators, respectively; the latter was obtained via the hydrogenation reduction of the former. Both the triblock copolymers had the same architecture, the identical composition and close molecular weights. In spite of the structural resemblance of both the triblock copolymers, the formation mechanisms of the nanophases in the thermosets were quite different. It was found that the formation of nanophases in the thermosets containing PCL-b-PBS-b-PCL followed a reaction-induced microphase separation mechanism whereas that in the thermosets containing PCL-b-PEEES-b-PCL was in a selfassembly manner. The different formation mechanisms of nanophases resulted in the quite different morphologies, glass transition temperatures (Tg's) and fracture toughness of the nanostructured thermosets. 2014 Elsevier Ltd. All rights reserved.