We report an investigation of the influence of block copolymer architectures on formation of nanophases in epoxy thermosets via reaction induced microphase separation approach. Toward this end, three binary block copolymers composed of polystyrene (PS) and poly(epsilon-caprolactone) (PCL) were synthesized via the combination of ring opening polymerization (ROP) and atomic transfer radical polymerization (ATRP). These block copolymers possess PS-b-PCL diblock, PS-b-PCL-b-PS triblock, and PCL-b-PS-b-PCL triblock architectures; they were carefully controlled to have the identical composition and overall molecular weights. It was found that the block copolymers with different architectures in epoxy thermosets displayed quite different reaction induced microphase separation behavior as evidenced with the results of atomic force microscopy (AFM), small angle X-ray scattering (SAXS), and dynamic mechanical thermal analysis (DMTA). The morphological transition from spherical to cylindrical to lamellar nanophases occurred with increasing the content of the block copolymer in the thermosets containing PS-b-PCL diblock copolymer. In the thermosets containing PS-b-PCL-b-PS triblock copolymer, unilamellar and multilamellar nanophases were formed depending on the content of the triblock copolymer. In contrast, the macroscopic phase separation occurred in the thermosets containing PCL-b-PS-b-PCL triblock copolymer The behavior of nanophases in these thermosetting blends have been accounted for the demixing behavior of the miscible blocks (viz. PCL) during the reaction induced microphase separation and the influence of copolymer architectures on the morphologies of PS microdomains.