Element　doping　is　an　effective　method　to　improve　the　performance　of　ZnO　varistors.　Previous　studies　mainly　focused　on　the　variation　of　microstructures　and　Schottky　barriers.　In　this　study,　the　effects　of　Co　dopant　on　electrical　properties　are　investigated　from　the　aspect　of　multiscale　defect　structures,　including　intrinsic　point　defects,　the　heterogeneous　interface　of　depletion/intergranular　layers,　and　interface　states　at　grain　boundaries.　Combining　with　analysis　of　phase　composition　and　energy　dispersive　spectroscopy,　it　is　found　that　Co　tends　to　dissolve　into　ZnO　grains　when　slightly　doped.　It　substitutes　Zn2+　with　the　same　valence　and　affects　little　on　densities　of　donors.　Segregation　of　Co　at　grain　boundaries　would　result　in　the　formation　of　spinel　phase　Co(Co4/3Sb2/3)O4　and　transformation　of　the　intergranular　phase　from　α-Bi2O3　to　δ-Bi2O3.　Meanwhile,　densities　of　point　defects　are　indirectly　affected　by　oxygen　ambient　during　sintering,　resulting　in　abnormal　variation　of　grain　resistivity.　And　interface　states　are　enhanced,　leading　to　improved　barriers　at　grain　boundaries.　Therefore,　reduced　leakage　current,　enhanced　grain　resistivity,　and　improved　non-linear　coefficient　in　Co-doped　ZnO　varistor　blocks　are　understood　from　the　underlying　multiple　defect　structures.　This　presents　a　potential　approach　to　explore　short-term　performance　and　long-term　stability　of　ZnO　varistors　from　the　aspect　of　defect　responses.　©　2020　American　Institute　of　Physics　Inc..　All　rights　reserved.