The surface of fumed nano‐silica (nano‐SiO2) was treated with an alkyl silazane to change from hydrophilic surface to hydrophobic, and the surface of aluminum trihydroxide (nano‐ATH) was treated with a vinyl silazane to improve electrical properties (high voltage direct current HVDC breakdown strength and tracking resistance) in silicone nanocomposites. They were prepared by mixing liquid silicone resin with the modified nano‐SiO2 and the modified nano‐ATH, and the effects of the modified nano‐ATH particles on the electrical properties were studied in order to apply them to heavy electric equipment. The surface modifications were confirmed by thermogravimetric analysis and contact angle analysis. As the nano‐ATH content increased, (±)HVDC breakdown strength increased and tracking resistance was greatly improved. As the nano‐ATH content increased, the initial generation of leakage current was delayed, and the occurrence frequency and the intensity of the leakage current decreased. The erosion weight due to the tracking also decreased.