The stability of ZnO-Pr$_{6}$/O$_{11}$-CoO-Er$_{2}$/O$_3$based varistors was investigated with Er$_{2}$/O$_3$additive content of the range 0.0 to 2.0 mol%. All varistors sintered at 130$0^{circ}C$ exhibited the thermal runaway within short times even under weak d.c. stress. As a result these varistors were completely degraded. On the contrary the stability of varistors sintered at 135$0^{circ}C$ was far better than that of 130$0^{circ}C$. In particular the varistors added with 0.5mol% Er$_{2}$/O$_3$ which the nonlinear exponent is 34.83 and the leakage current is 7.38 $mutextrm{A}$ showed a excellent stability which the variation rate of the varistors voltage the nonlinear coefficient and the leakage current is below 1%, 2%, and 3.5% respectively even under more severe d.c. stress such as (0.80 V$_{1mA}$9$0^{circ}C$/12h)+(0.85 V$_{1mA}$115$^{circ}C$/12h)+(0.90 V$_{1mA}$12$0^{circ}C$12h) Consequently it is estimated that the ZnO-0.5 mol% Pr$_{6}$/O sub 11/-1.0 mol% CoO-0.5 mol% Er$_{2}$/O sub 3/ based varistors will be used to develop the advanced Pr$_{6}$/O$_{11}$-1.0 mol% CoO-0.5 mol% Er$_{2}$/O$_{3}$ based varistors will be used to develop the advanced Pr$_{6}$/O$_{11}$-based ZnO varistors having the high performance and stability in future. future.ure. future.