In order to make a low-voltage ZnO varistor of uniformly large-grained microstructure, a pure ZnO powder compact was first sintered at 1500 °C. The abnormal grain growth that occurred during the sintering of the ZnO resulted in a relatively uniform matrix of very large grains. Subsequent heat-treatment in contact with a Bi 2O 3 powder mixture resulted in the formation of a low-voltage ZnO varistor.