ZnO-Bi2O3 based varistor ceramics and ZrO2 doped ZnO varistor ceramics are sintered by using different sintering time, and the electrical properties and microstructure of the varistor ceramics are studied in this paper.The results indicate that with increasing sintering time, the density and the nonlinear coefficients of ZrO2 doped ZnO varistor ceramics first increase and then decrease, however, the leakage current and the threshold voltage first decrease and then increase.The grain size of ZrO2 doped ZnO varistor ceramics is smaller than the grain size of ZnO-Bi2O3 based varistor ceramics.The most probable reason is that Zr atomic radius is close to Zn atomic radius.So Zr is dissolved in solid solution with ZnO grains and inhibits the growth of ZnO grain.The grain growth exponent (n) of ZrO2 doped ZnO varistor ceramics is larger than ZnO-Bi2O3 based varistor ceramics.The growth activation energy (Q) of ZnO-Bi2O3 based varistor ceramics prepared by ball milling is large, Q=231±27 KJ/mol.Maybe this is because ZrO2 and spinel particle pin the ZnO grain-boundaries creating a drag mechanism that reduces the grain boundary mobility and increases the apparent activation energy.