A rubber-modified epoxy resin is widely used as the adhesive and as the matrix material for fiber composite materials. The structural integrity of composite materials depends on fracture toughness of the matrix resin. In this study, we investigated the fracture toughness and the damage zone around a crack tip of the rubber-modified epoxy resin whose volume fractions of rubber(CTBN 1300$ imes$8) were O％, 5％ and 15% under several loading speeds. The fracture toughness(K$_{IC}$) and the fracture energy(G$_{IC}$) were measured using 3-point bending specimens. The 4-point bending specimens were also used to observe damage zones in the vicinity of crack tips for the modified resins. The results show that the fracture toughness and the size of damage zones at 5％ and 15％ rubber contents decreased with increase in loading speed.eed.