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We studied the effects of indium doping on the microstructure and electrical properties of ZnO varistors co-doped with Al 2 O 3 and Ga 2 O 3 . Scanning electron microscopy, current–voltage testing, capacitance–voltage testing, and X-ray diffraction were performed. The results showed that the leakage current of sintered varistors decreased as the indium dopant concentration was increased. The average grain size decreased slightly with increasing indium content, which improved the voltage gradient. A certain amount of the doped In 3+ dissolved into the grains causing the grain resistance to decrease, which resulted in a low level residual voltage. The sintered samples with 0.024 mol% indium, 0.2 mol% aluminum, and 0.42 mol% gallium showed excellent overall performance, exhibiting a nonlinear coefficient of 58.8, a leakage current of 1.82 µA/cm 2 , a 1-mA residual voltage of 433.9 V/mm, and a residual voltage ratio of 1.53. The obtained co-doped ZnO varistors will provide better protection and stability in applications as metal oxide arresters for power systems. [ABSTRACT FROM AUTHOR]