The phase evolution, microstructure, and electrical properties of WO3-doped ZnO-Bi2O3-based varistors were investigated for different amounts x (0x1.60mol%) of the dopant. When x was less than 0.40, the dissolved W6+ in the -Bi2O3 acted as a donor in the grain boundaries and reduced the electrical properties of the ZnO varistors. However, when x was 0.40mol%, which meant an amount of WO3 equal to that of Bi2O3, the electrical properties dramatically increased, which means the W6+ donor effect is removed at the grain boundaries because a new Bi2WO6 phase was formed in the grain-boundary regions. The Bi2WO6 phase has high oxygen conductivity at high temperatures; it transfers more oxygen to the grain boundaries in order to further enhance the electrical properties. For x values higher than 0.40 (i.e., an addition of WO3 that is greater than the content of Bi2O3), the electrical properties were steadily reduced in comparison to the composition with x=0.40. This could be explained by the reduced amount of Co, Mn, and Al at the grain boundaries and in the ZnO grains as a result of their incorporation into the ZnWO4 phase. The electrical properties of the ZnO grains and the grain boundaries were in agreement with the results of the impedance spectroscopy analysis.