Electric breakdown strength and dielectric properties of epoxy/micro-silica and epoxy/micro-alumina composites were studied in order to develop an insulation material for high voltage application. Electrical insulation breakdown strength test were carried out in sphere-plate electrodes and the data were statistically estimated by Weibull method. Dielectric properties was also tested for the specimen with 300 μm thick and 30 mm diameter. Electrical breakdown strength of epoxy/micro-silica was higher than that of neat epoxy, and that for epoxy/micro-alumina was also higher than that of neat epoxy. And electrical breakdown strength of epoxy/micro-silica composite was higher than that of epoxy/micro-alumina composite. As silica or alumina content increased, dielectric constant and tanδ increased.