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The aim of this study is to improve both thermal conductivity and electrical insulation properties of epoxy based composites using nanocomposite techniques. This paper deals with the effects of adding nano-scale alumina and silica fillers on thermal conductivity and dielectric strength of epoxy/alumina micro-composites. Two kinds of specimens were prepared with containing nano-fillers as a nanocomposite or a nano/micro composite. Also, two kinds of specimens without fillers or with 30 vol% of micro-scale alumina fillers as base materials. Average particle sizes of the alumina and silica nano-fillers used were 14 nm and 12 nm, respectively. The micro-fillers used were alumina particle of spherical shape, and its average particle size was about 10 μm. Nano-filler content was 5 wt% in each specimen. Thermal conductivity was evaluated by the laser flash method. AC dielectric strength was measured by a sphere-plane electrode system immersed in insulating liquid. Cracked surfaces were observed with the scanning electron microscope to check the state of nano-fillers dispersion. It was elucidated that an improvement effect of each nano-filler addition is seen on both thermal conductivity and dielectric strength.