The dc electrical stability of ZnO varistors was effectively improved by controlling the composition proportion of Bi2O3 and SiO2 as well as annealing condition. The microstructure, current-voltage (I-V) property, and dc degradation characteristics are significantly affected by the molar ratio of Bi2O3 to SiO2 in composition. It is found that a phase transition from beta-Bi2O3 (with dissolved Si) to Bi12SiO20, with a volume contraction of 5.88%, occurred after annealing at 850 degrees C. The formed Bi12SiO20 plays an important role in improving the electrical stability by inhibiting the oxygen-desorption at the grain boundary.