Voltage-current characteristics of ZnO surge arresters are highly dependent on temperature in the low-conduction region. ZnO surge arresters experience thermal runaway when the temperature exceeds the acceptable limit. This phenomenon is associated with the increase of resistive leakage current due to degradation. This paper presents the thermal and electrical characteristics of ZnO arrester blocks under the power frequency AC operating voltages. The leakage currents of ZnO arrester blocks were measured over a period of the time. The temperature profiles of ZnO arrester blocks were observed by the infrared camera. The degradation and the thermal runaway of the ZnO arrester blocks are closely related to the temperature limit which determines the heat generation and dissipation. As the leakage current was 0.5 mA, the amount of heat generation is lower than that of heat dissipation. So temperature of the ZnO arrester blocks was rarely changed. On the other hand, as the leakage current was 0.7 and 1 mA, the amount of heat generation was greater than that of the heat dissipation. The temperature of ZnO arrester blocks gradually increases and the test specimen experiences thermal runaway. At that time the temperature located at the center of ZnO arrester blocks string was higher compared to that of ZnO block at both edges. (C) 2009 Elsevier B.V, All rights reserved.