To disperse RTV-silicone elastomer as fine particles in a bisphenol-A-type epoxy resin, silicone-methylmethacrylate graft copolymer was added as a compatibilizer. The molecular weight of the silicone segment and the MMA segment between silicone branches in the graft copolymer strongly affected the effectiveness of compatibilizer. It was revealed that the graft copolymer which acts as a more effective compatibilizer was more highly concentrated at the interface between the silicone-dispersed phase and the epoxy matrix. The interfacial tension around the silicone-dispersed phase decreased with the formation of the interphase which is mainly composed of the graft copolymers. The diameter of the dispersed phase decreased with a decrease in the interfacial tension. The fracture toughness of the modified resins increased with a decrease in the diameter of the silicone phase.