In this study, we synthesized poly(ethylene oxide)-block-poly(epsilon-caprolactone)-block-polystyrene (PEO-b-PCL-b-PS) triblock copolymer via the combination of ring-opening polymerization (ROP) and atomic transfer radical polymerization (ATRP). The ABC triblock copolymer was incorporated into epoxy to access the nanostructured thermosets. It is found that the nanophases of the epoxy thermosets can be modulated by using different hardeners. While cured with 4,4'-methylenebis(2-chloroaniline), the thermosets displayed the long-ranged ordered nanostructures in which the spherical nanophases were arranged into body-centered cubic (bcc) lattice at the compositions investigated. While 4,4'-diaminodiphenylsulfone was used as the hardener, the thermosets displayed the lamellar nanostructure. The formation of nanostructures in the thermosets has been evidenced by atomic force microscopy and small-angle X-ray scattering. The morphological transition from spherical to lamellar nanophases has been interpreted in terms of the microphase separation of different subchains of the ABC triblock copolymer out of the epoxy-amine matrix during the curing reactions owing to the dependence of miscibility of epoxy networks with PCL subchain of the triblock copolymer on types of hardeners. The kinetics of curing and microphase separation shows the tandem reaction-induced microphase separation occurred while DDS was used as the hardener, which gave rise to the formation of lamellar nanostructures in the epoxy thermosets containing the ABC triblock copolymer.