The electrical properties of varistors consisting of Zn-Pr-Co-Cr-Er oxides were investigated in the Er$_2$O$_3$content range of 0.0 to 2.0 mol%. the varistors without Er$_2$O$_3$ exhibited a relatively low nonlinearity, which was 14.24 in the nonlinear exponent and 21.47 $mu$A in the leakage current. However, the varistors with Er$_2$O$_3$ sintered at 1335$^{circ}C$ for 1h exhibited very high nonlinear exponent of 70, in particular, reaching a maximum value of 78.05 in 2.0 mol% Er$_2$O$_3$, and those sintered at 1335$^{circ}C$ for 2h exhibited the nonlinear exponent close to 50, in particular, reaching a maximum value of52.76 in 0.5 mol% Er$_2$O$_3$. The others except for 0.5 mol% Er$_2$O$_3$-added varistors exhibited very high instability resulting in a thermal runaway within a short time, even a weak DC stress. Increasing soaking time decreased the nonlinearity, but increased the stability. The varistors containing 0.5mol% Er$_2$O$_3$ sintered for 2h exhibited excellent stability, in which the variation rate of the varistor voltage and nonlinear exponent was -1.70% and -7.15%, respectively, under more severe DC 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)+(0.95 V$_{1mA}$/1$25^{circ}C$/12h)+(0.95 V$_{1mA}$/15$0^{circ}C$/12h).TEX>/12h).