Bi2O3 is an important additive in the composition of ZnO varistors. However, the effect of Bi2O3 on novel SnO2 based varistors is still unclear. This paper reports the nonlinear electrical characteristics of (98.95-x)mol%SnO2-0.5mol%CoO-0.05mol%Nb2O5- xmol% Bi2O3 (x = 0, 0.05, 0.1, 0.5 mol%) system, and the influence of sintering temperature is also investigated. The addition of Bi2O3 can improve the electrical nonlinearity of SnO2-CoO-Ta2O5 based varistors, this may be due to the higher barrier height and barrier voltage. The optimum Bi2O3 doping level is between 0.1-0.5mol% and the optimum sintering temperature is 1350°C.