The effect of Cr2O3 additive on the leakage conduction and threshold voltage characteristics of bismuth-based ZnO ceramic varistor was studied. The leakage conduction in the voltage range below the threshold voltage increased with increasing Cr2O3 concentration and is attributed;to the Schottky barrier height. It was found that the increases in the apparent threshold voltage were associated with the lowered donor concentration in the depletion region of the ZnO grain. These results were obtained by measuring C-V characteristics, the breakdown voltage at the current density of 10(-3) Acm(-2) and the microstructure of ZnO ceramics.