The amphiphilic triblock copolymer poly(epsilon-caprolactone)-b-polybutadiene-b-poly(c-caprolactone) (PCL-PB-PCL) was synthesized via the ring-opening polymerization of E-caprolactone in the presence of a hydroxyl-terminated polybutadiene (HTPB), which was catalyzed by stannous octanoate [Sn(Oct)(2)]. The amphiphilic triblock copolymer was further used to prepare the nanostructured epoxy thermosets. The in situ polymerization of epoxy monomers in the presence of PCL-PB-PCL started from the homogeneous solutions composed of the block copolymer and the monomers of epoxy at the temperature above the upper critical solution temperature (UCST) of diglycidyl ether of bisphenol A (DGEBA) and HTPB blends. The transmission electronic microscopy (TEM), small-angle X-ray scattering (SAXS), and atomic force microscopy (AFM) showed that the nanostructured thermosets were successfully obtained. Depending on the content of the block copolymer in the thermosets, the PB domains can display spherical and interconnected nanoobjects with the size of 10-20 nm. It is judged that the nanostructures are formed on the basis of the mechanism of polymerization-induced rnicrophase separation, which is in a marked contrast to the approach in which some equilibrium self-organized structures were preformed and the microphases were fixed via subsequent curing reaction.