The electrical characteristics of zinc oxide (ZnO) varistors are closely correlated with the state of impurities, such as Bi2O3, at grain boundaries. To clarify the distributions of these impurities, the fracture surface of a BiMnCoSbSi-added ZnO varistor was observed and characterized using scanning electron microscopy, energy dispersive X-ray spectrometry, and back-scattered electron detection. It was found that depositing Bi2O3 as an additive yielded two different shapes, spot like and sheetlike, on the surface of the grain boundary fracture for the BiMnCoSbSi-added ZnO varistor. By the dissolution of SiO2 in Bi2O3, the surface tension of Bi2O3 decreased and the sheetlike Bi2O3 deposit increased, whereas the spot like Bi2O3 deposit decreased. Moreover, when the sample was annealed at 700 degrees C, a compound of Bi and excess Zn formed a uniform thin deposit or moved to triple points, since the surface free energy (surface tension) decreased. The main factor responsible for the improvement in resistance to electrical degradation by the addition of SiO2 or by annealing is the increase in the surface area of the ZnO grains by the sheetlike deposit of Bi2O3, since this deposit hinders the movement of oxide ions or Zn2+ ions across the grain boundaries under an applied voltage.