A thermoplastic polymer modification was tried for the purpose of simultaneously improving the heat resistance and toughness of a cured epoxy resin. The thermoplastic polymer was formed during the cure process of the epoxy resin in order to avoid any increase in the viscosity of the resin at the time of mixing. When 24 wt% of the phenylmaleimide/benzyl methacrylate/styrene monomers with a molar ratio composition of 5/5/3 was added to epoxy and phenol resins, the resin viscosity at mixing fell to about 1/8 of an unmodified resin. The epoxy and phenol resins were cured, forming a thermoplastic polymer by radical polymerization of these monomers. The glass transition temperature (T-g) of the cured resin was improved by about 10 degreesC and the value of the fracture toughness was also improved by about 2.5 times that of the unmodified resin. This is attributed to the formation of the semi-interpenetrating polymer networks (semi-IPN) composed of the epoxy network and linear thermoplastic polymer.