Based on the previously established knowledge about the micro-mechanical deformations occurring during the fracture of glass bead filled epoxies, the major energy dissipation mechanisms are investigated. Correlation studies between the fracture toughness of composites and the size of micro-mechanical deformation zones (or areal density of deformation) are used to assess the contributions of the deformations to toughening. Among the deformations found in the fracture of glass bead filled epoxies, i.e. micro-shear banding, debonding of glass beads/diffuse matrix shear yielding, and step formation, micro-shear banding is established as the major and most effective toughening mechanism. In terms of this mechanism, the negligible effect of surface treatments of glass beads on the fracture toughness of glass bead/thermoset composites can be explained successfully This mechanism is expected to give more detailed and fundamental understanding of inorganic particle toughening than the crack front bowing mechanism.