The Voronoi network can be used to effectively simulate the microstructure of ZnO varistors. The nonuniformity in microstructure of simulated ZnO varistor can be changed by setting different disorder degree of Voronoi network. In the region of disorder degree larger than 3 where the simulated microstructures are similar to those the actual ones of ZnO varistors, a chaotic phenomenon exists in the microstructure characteristics. This chaotic property can simulate the original behavior of nonuniformity of electrical characteristics caused by microstructures of ZnO varistors.