ZnO varistors containing 5.0 at% Co3O4 and Pr6O11, ranging from 0.1 to 1.0 at%, were sintered at 130$0^{circ}C$ and 135$0^{circ}C$. The I-V characteristics and nonlinear coefficients of the specimens were investigated with respect to Pr addition and sintering temperature. In general the specimens sintered at 130$0^{circ}C$ showed better varistor characteristic than those fired at 135$0^{circ}C$, which seemed to be related with the liquid phase formation during sintering. The barrier heights obtained from C-V relations, 0.29-1.36 eV, were different from those acquired using resistivity-temperature plots measured at low voltage per grain boundary. Therefore the estimation of potential barrier heights using C-V relations is better suited for the specimens prepared in this study. The carrier densities obtained using C-V relations were ~1018 cm-3.