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The nonuniformity of electrical characteristics of grain boundaries in ZnO varistors was systematically analyzed. The high nonuniformity exists in barrier voltages and nonlinearity coefficients in different grain boundaries. The barrier voltages have normal distributions, only a few grain boundaries were electrically active, and the grain boundaries can be simply classified into good, bad, and ohmic ones according to the electrical characteristics of grain boundaries. The average barrier voltage is equal to 3.3 V by the direct method, but it is only 2.3 V by the indirect method. There is a high difference between the barrier voltages by direct and indirect measurement methods. These few good grain boundaries are responsible for the good varistor effect, and control the leakage current of ZnO varistor at low values of applied voltage. The Al2O3 dopants affect the electrical characteristics of grain boundaries by changing the electron status in grain boundary and intragrain.