Polystyrene block-poly(epsilon-caprolactone) block poly(n butyl acrylate) (PS-b-PCL-b-PBA) triblock copolymer was synthesized through the combination of atom transfer radical polymerization, copper catalyzed Huisgen 1,3-dipolar cycloaddition and ring opening polymerization The PS b-PCL-b PBA ABC triblock copolymer was incorporated into epoxy to access the nanostructures in the thermoset The microphase separated morphology was investigated by means of atomic force microscopy (AFM), small-angle X-ray scattering (SAXS) and dynamic mechanical thermal analysis (DMTA) It was found that depending on the concentration of the triblock copolymer in the thermosets several kinds of nanodomains were formed and they were arranged in lamellar lattice The formation of the nanostructures was ascribed to the tandem reaction induced microphase separation of PBA and PS blocks in the thermosetting blends The investigation of the model binary thermosetting blends showed that the phase separation of PBA occurred at the conversion much lower than that of PS It is proposed that the PBA nanophases were formed prior to the PS nanophases in the thermosetting blends and the microdomains of PBA subchains could behave as the template for the demixing of PS blocks The coupling of the two stage reaction induced microphase separation exerted a profound impact on the formation of nanostructures in the epoxy thermosets containing the ABC triblock copolymer Thermal analysis shows that with the formation of the nano structures in the thermosets a part of poly(epsilon caprolactone) subchains were demixed from epoxy matrix, the fractions of demixed PCL blocks have been estimated according to the T-g-composition relation of the model binary blends of epoxy and PCL