In order to improve the heat resistance of a cured epoxy resin together with reducing the viscosity of the resin composition, an epoxy resin was cured with a curing agent formed from the radical copolymerization of vinyl monomers during the cure process of the epoxy resin. N-phenylmaleimide and p-acetoxystyrene were used as vinyl monomers of the curing agent. The epoxy resin was cured by the insertion reaction of the ester group of the in situ polymerized curing agent and the epoxy group of the epoxy resin. In the cure system of the epoxy and the phenol resins, reduction of the viscosity of the resin composition was achieved by replacing some or all of the phenol resin with these monomers. When all phenol resins were replaced by these monomers, the viscosity of resin composition (0.01 Pa s at 70 degreesC) decreased by about 1/2000 compared with that of the system with only phenol resin (21 Pa s at 70 degreesC). The glass transition temperature (T-g) of the cured resin with no phenols was 174 degreesC, an improvement of 17 degreesC compared with that of the system cured with only phenol resin. The flexural strength of the new resins remained unchanged.