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- The influence of microstructure nonuniformity on the electrical characteristics of ZnO varistors was analyzed with the added amount of Al2O3 dopants. Al2O3 doping can effectively inhibit grain growth. When Al2O3 content is in the range between 0-0.1%, the average grain size and the standard deviation decrease quickly and the grain growth is strongly inhibited. Therefore, it is possible to increase the microstructure uniformity by accurate addition of Al2O3 to the ZnO varistor. The breakdown voltage increases with the decrease of standard deviation. The greater the uniformity of the ZnO varistor means the higher the global breakdown voltage. The Al 2O3 dopants having about 0-0.023 wt% content can effectively improve the voltage ratio, and the voltage ratio reaches a minimum value of 2.32 at an Al2O3 content of 0.005 wt%.