In this study, acrylic tri-block-copolymers of different compositions and constituents were used to modify an epoxy resin. In epoxy the block-copolymers self-assembled to generate different nanostructured phases. Spherical micelles and vesicles were formed due to preferential interactions among constituents of the blends. The effect of modifier concentration and morphology on the resulting mechanical properties was investigated. At a modifier content of 5 wt% a more than threefold increase in strain energy release rate was observed relative to the unfilled epoxy for the block-copolymer/epoxy systems studied. This improvement was accompanied by slight reductions in tensile stiffness, strength and glass transition temperature of developed epoxy blends. Microstructural studies revealed that during crack propagation, vesicle and micelle structures respectively underwent debonding and cavitation followed by shear yielding of the substrate matrix. POLYM. ENG. SCI., 54:1047-1055, 2014. (c) 2013 Society of Plastics Engineers