A polystyrene-b-poly(ethylene oxide) (PS-b-PEO) diblock copolymer was synthesized via the atom transfer radical polymerization (ATRP) of styrene with mono-2-bromoisobutyryl-terminated PEO[PEO-OOCCBr(CH3)(2)] as a macroinitiator, and the polymerization was mediated by copper(I) bromide (CuBr) and 2,2'-bipyridine (BPY). The PS-b-PEO diblock copolymer was used to incorporate into epoxy resin to afford the nanostructured epoxy thermosets. Both atomic force microscopy (AFM) and small-angle X-ray scattering (SAXS) showed that the long-range ordered nanostructures were obtained via the approach of in situ polymerization in the presence of the diblock copolymer. It was found that the nanophases of PS were arranged into a simple cubic symmetry lattice while the content of diblock copolymer was 40 wt %. In view of the difference in miscibility and phase behavior for the blends of the subchains (i.e., PS and PEO) of the diblock copolymer with epoxy after and before curing, the formation of the ordered nanostructures was judged to be via the mechanism of reaction-induced microphase separation, which is in marked contrast to the mechanism of self-organization of block copolymers together with the subsequent fixation of the self-organized structures.