Vaporization of Bi2O3 in microwave-sintered ZnO varistors is discussed in this study. The Bi2O3 vaporization of ZnO varistors sintered by a conventional electric furnace is also studied for comparison. The results show that the Bi2O3 vaporization in microwave-sintered ZnO varistors is more homogenous from the surface to the inside of the sample, which results from the special thermal gradient inside the microwave-sintered samples, and we also find out that the Bi2O3 vaporization directly affects the electrical properties of ZnO varistors. Microwave-sintered samples exhibit more excellent electrical properties than the conventional ones because the homogenous Bi2O3 vaporization leads to more uniform microstructures.