We are aiming at the development of epoxy composite materials with both high thermal conductivity and sufficient voltage endurance. A target is set as 10 W/m/K with 5 kV withstand voltage for power electronics applications. In order to achieve high thermal conductivity for polymers such as epoxy, it is a common method to fill inorganic micro particles with high thermal conductivity, but such composites prepared in this way shows lower endurance voltage. To compensate such negative performance, it is considered to be a good way to add nano sized inorganic particles. After many trials were made, good results were obtained for a composite material with two modes of BN and alumina and surface treated nano silica. It has thermal conductivity larger than 5- 10 W/m/K with reasonable withstand voltage.