This article is a report on the effect of Bi2O3 addition on microstructure and electrical properties of ZnOV2O5MnO2Nb2O5 varistor ceramics. The average grain size increased from 5.6 to 7.2 mu m as the Bi2O3 amount increased. Increasing the amount of Bi2O3 decreased the breakdown field from 4874 to 2205 V/cm. The ceramics doped with 0.025 mol% Bi2O3 exhibited a surprisingly high nonlinear coefficient (a = 60) and very low leakage current density (JL = 20 mu A/cm(2)). In addition, when the Bi2O3 amount increased, the donor concentration increased in the range of 2.38 x 1017-8.17 x 1017 cm(-3) and the barrier height exhibited the highest value (1.12 eV) at 0.025 mol% Bi2O3.