The microstructure, electrical properties, and stability of Pr6O11-based ZnO varistors, which are composed of ZnO-Pr6O11-CoO-Er2O3 systems, were investigated with Er2O3 additive content. The density of ceramics was in the range of 84-88% of TD at 1300 degreesC and 93-98% of TD at 1350 degreesC, and greatly affected the stability. Most of the added-Er2O3 were segregated at nodal points. The varistors with 0.5 mol% Er2O3 sintered at 1300 degreesC exhibited the best nonlinear current-voltage characteristics, which the nonlinear exponent is 52.8 and the leakage current is 9.8 muA. All the varistors sintered at 1300 degreesC, even under relatively weak stress, exhibited the thermal runaway within short time in order of high leakage current. On the contrary, the stability of varistors sintered at 1350 degreesC exhibited far higher stability than that at 1300 degreesC. Particularly, the varistors with 0.5 mol% Er2O3 exhibited not only relatively good nonlinear current-voltage characteristics, which the nonlinear exponent is 34.8 and the leakage current is 7.4 muA, but also excellent stability, which the variation rates of varistor voltage, nonlinear exponent, and leakage current are below 1%, 3%, and 3%, respectively, even under more severe stress such as (0.80 V-1 (mA)/90 degreesC/12 h) + (0.85 V-1 (mA)/115 degreesC/12 h) + (0.90 V-1 (mA)/120 degreesC/12 h).