The morphology, fracture toughness and mechanical properties of an anhydride-cured diglycidylether of bisphenol-A epoxy polymer modified with poly(methyl methacrylate)-b-poly(butylacrylate)-b-poly(methyl methacrylate) (MAM) have been investigated. The addition of three different MAM triblock copolymers(M22N, M52N and M52) to the epoxy polymer gives two different microstructures. A nanostructure with well-dispersed worm-like micelles (or a bicontinuous gyroid structure if the micelles are connected into a network) was obtained using M22N. The addition of M52N or M52 gives dispersed micron-size particles in the epoxy matrix for <= 7 wt% MAM, and a co-continuous microstructure at higher MAM contents. These triblock copolymers toughen the epoxy polymer significantly, with only slight reductions in the mechanical and thermal properties of the epoxy polymer. The maximum values of fracture toughness and fracture energy (1.22 MPa m(1/2) and 450 J/m(2), respectively) were measured using 12 wt% M22N, which is an increase of 100 and 350%, respectively, compared with the unmodified epoxy. The M52- and M52N-modified materials show a maximum toughness when a co-continuous microstructure is formed. The potential toughening mechanisms are identified and discussed.