An inorganic filling agent, ATH (alumina trihydrate) was used to induce inorganic-organic coupling by mixing with stearic acid, acryl silane, vinyl silane as surface treatment agents in order to apply as a high voltage insulating material. Volumetric resistivity was shown to vary with surface treatment agents, and the highest value was obtained in case of the mixture with vinyl silane. The dielectric breakdown intensity was shown to decrease gradually and saturate to a stable value, possibly due to the increase of cross link density in the vinyl radicals introduced to silica surface, resulting in stable dielectric breakdown intensity in the final stages. Tracking and flame retardant properties were also shown to be the best among the samples investigated in this study.