Epoxy resin often suffers lower dielectric breakdown strength if micro-fillers are loaded. It might be possible to raise the once lowered breakdown strength, if nano-fillers are added. In this paper, such performances were investigated on model specimens consisting of alumina (Al2O 3). First of all, PD (partial discharge) behavior on the surfaces of samples was clarified in the insulation system consisting of a flat copper electrode attached on an epoxy substrate. Then, on the basis of finding, the breakdown strengths were investigated by using a sphere to sphere electrode structure with a flat sample inserted. Several materials consist of neat epoxy, 5%wt nano-Al2O3/epoxy composite, 60%wt micro-Al 2O3/epoxy composite and 2%wt nano-60% wt\micro-Al 2O3/epoxy composite. It was found that nano-micro-Al 2O3/epoxy composite is higher in both dielectric strength and PD resistance than micro-Al2O3/epoxy composite.