In this study we aims to examine the effects of $Co_3O_4$ and NiO doping on the defects and electrical properties in ZnO-$Bi_2O_3-Sb_2O_3$ (Sb/Bi=0.5) varistors. It seemed to form ${Zn_i}^{{cdot}{cdot}}$(0.20 eV) and ${V_o}^{cdot}$(0.33 eV) as dominant defects in Co and Ni co-doped ZBS system, however only ${V_o}^{cdot}$ appeared in Co- or Ni-doped ZBS. Even though the same defects it was different in capacitance (1.5~4.5 nF) and resistance ($0.3{sim}9.5k{Omega}$). The varistor characteristics were improved with Co and Co+Ni doping (non-linear coefficient, ${alpha}$= 36 and 29, relatively) in ZBS. The various parameters ($N_d=1.43{sim}2.33{ imes}10^{17}cm^{-3}$, $N_t=1.40{sim}2.28{ imes}10^{12}cm^{-2}$, ${Phi}b$=1.76~2.37 V, W= 98~118 nm) calculated from the C-V characteristics in our systems did not depend greatly on the type of dopant, which were in the range of a typical ZnO varistors. It should be derived a improved C-V equation carefully for more reliable parameters because the variation of the varistor capacitance as a function of the applied dc voltage is depend on the defect, frequency, and temperature.