The microstructure and electrical properties of praseodymium-based zinc oxide varistors were investigated at various cobalt oxide contents in the range of $0.5{~}5.0 mol\%$. The ceramic density increased in the range of $5.25{~}5.55 g/cm^3$ with increasing cobalt oxide content. The varistor doped with cobalt oxide of $1.0 mol\%$ exhibited the highest nonlinearity, with 66.6 in nonlinear exponent and 1.2 $mu$A in leakage current. The donor concentration, density of interface states, and tamer height were in the range of $(1.06{~}1.69){ imes}10^{18}/cm^3$, $(3.11 {~}3.56){ imes}10^{12}/cm^2$, and 0.80${~}$1.07 eV, respectively.