This paper describes the results of investigation made into the performance of insulators covered with snow or ice. The AC and DC withstand voltages of snow-covered insulators were characterized by the conductivity of water melted from the snow. The AC withstand voltage of ice-coated insulators was characterized by the length of icicles and conductivity of the water melted from the ice. And the switching impulse flashover voltage of snow-covered insulator strings was almost proportional to gap length between the arcing horns or the insulator strings. Flashover voltage of a 75% snow-covered insulator string, which means the worst case, was 40 ?65% of the positive wet flashover voltage of the clean insulator string.