We report our finding of an optimal length scale for toughening of epoxies using spherical micelles formed by block copolymers. The amphiphilic diblock copolymer poly(hexylene oxide)-poly(ethylene oxide) (PHO-PEO) with 30 wt % PEO self-assembled to form spherical micelles in a bisphenol A epoxy resin with a phenol novolac hardener. We systematically increased the size of the spherical micelles from 20-30 nm to 0.5-10 mu m by swelling their PHO core using PHO homopolymer. Although all the blends were tougher than the unmodified epoxy, the largest enhancement of fracture resistance was measured in blends containing 0.1-1 mu m spherical inclusions. This enhanced toughness was correlated with plastic deformation by shear banding in tensile test and greater roughness of the fracture surface. Smaller micelles neither induced plastic deformation nor contributed to surface roughness significantly whereas larger micelles acted as local defects resulting in early failure. These findings provide a framework in assessing the toughening effects of blended block copolymers on epoxy resins. (C) 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polyrn Phys 47: 1125-1129, 2009