A rubber-modified epoxy resin is widely empolyed as the base of adhesive compositions asnd as the matrix material of glass and carbon-fiber composites. In the present study, we investigated the toughening mechanism of the rubber-modified epoxy resins with different rubber contents. The damage zone and rubber particles around a crack tip of a damaged specimen just before fracture were observed using a polarization microscope and an atomic force microscope (AFM). Both the fracture energy(G$_{IC}$) and the size of damage zone increased with the rubber content below 15wt%. The size of the rubber particles could be qualitatively correlated with the G$_{IC}$ and the size of damage zone. The cavitation of the rubber particles inside the damage zone was observed, which was expected to be the main toughening mechanism of rubber particles. The stress which causes the cavitation of rubber particles was estimated by the Dugdale model.del.