The shortcoming of epoxy resin is the brittleness of this material though it shows excellent chemical, mechanical and electric properties. To improve fracture toughness of epoxy resin, rubbery materials that show high values in toughness but low values in glass transition temperature and mechanical properties, and thermoplastics that show high values in thermal and mechanical properties but relatively small increase in toughness were blended with epoxy. ATPEI-CTBN-ATPEI triblock copolymer, which consists of rubbery and thermoplastics blocks, was synthesized, and the triblock copolymer was blended with epoxy resin. The effects of parameters such as contents of the triblock copolymer, cure temperature, and contents of catalyst on the morphology of the blend systems were studied. From 30 wt% of the contents of the triblock copolymer, fracture toughness and impact energy absorption of the blend systems were increased significantly. This was due to the generation of nodular morphology in the system.