Abstract: CuO effects on the electrical properties of （Ni, Ta）-doped SnO₂ varistor material were inves-tigated. When CuO concentration increases from 0.50mol%to 1.5mol%, the varistor field increasessignificantly from 132V/mm to 234V/mm, the relative dielectric constants decreases greatly from 4663to 2701. With CuO concentration increases from 0.50mol%to 1.5mol%, the SnO₂ grain size decreases significantly from 18.8μm to 13.3μm, which heightens the varistor fields. The reduction of the grainsize attributes to the CuO segregation at grain boundaries hindering the SnO₂ grains from merging each other to grow larger. The grain-boundary defect barrier model was modified to illustrate the nature of the electrical nonlinearity for the material. And an analysis of an equivalent circuit was conducted. Theexperimental result has a good coincidence with which derived by equivalent circuit.