In this paper, the effects of boron and yttrium co-doping on the electrical properties of ZnO varistors were determined. The presence of both B3+ and Y3+ co-dopant improved the electrical performance of varistor ceramics. The nonlinearity coefficient α and potential barrier ϕ b were increased noticeably. The nonlinearity coefficient α was 73 and ϕ b was 2.75 eV, at the same time, a voltage gradient of 425 V/mm and leakage current of 0.9 μA/cm2 were obtained. This work provided a way to enhance the protective effects of ZnO varistor arresters for ultra-high voltage power transmission systems. • The nonlinearity coefficient α is 73 and φ b is 2.75 eV. • The voltage gradient of 425 V/mm and leakage current of 0.9 μA/cm2 were obtained with the B3+ and Y3+ co-dopanted recipe. [ABSTRACT FROM AUTHOR]