Epoxy resin is used as a material for electrical and electronics molding in various forms but its thermal conductivity must be controlled with various additives on account of its lower conductivity than metal or ceramics. Silicon dioxide (SiO2) and silica were selected as the reinforcement and diluent filler for epoxy resins, respectively. The optimum amount of reinforcement filler, SiO2, was 50 wt%. The thermal properties and thermal stability were observed according to silica ratio and particle size. An epoxy modified with a polyamide type hardener showed superior thermal conductivity to that modified with a cyclo-aliphatic amine type hardener. The thermal conductivity increased with increasing silica ratio and particle size. The thermal stability evaluation based on the particle size of silica was in the order of 14/18 mesh (1.00-1.16 mm) > 8/10 mesh (1.65-2.36 mm) > 28/35 mesh (0.42-0.59 mm). The optimum silica size of the diluent filler was 14/18 mesh (1.00-1.16 mm). An epoxy type resin transformer with excellent thermal properties and thermal stability could be designed when the mixing weight of epoxy resin was equal to that of the hardener.