ZnO　varistors　were　prepared　by　microwave　sintering　under　different　oxygen　partial　pressures.　The　temperature　profile　and　the　densification　behavior　in　different　atmospheres　were　investigated.　It　was　found　that　the　density　of　ZnO　varistors　during　sintering　was　the　key　factor　affecting　the　absorption　of　microwave　energy.　The　electrical　properties,　including　the　nonlinear　properties　and　capacitance-voltage　(C-V)　characteristics,　were　also　carefully　studied.　The　results　showed　that　the　oxygen　partial　pressure　has　significant　effects　on　the　electrical　properties　of　ZnO　varistors　by　changing　the　concentration　of　defects　through　a　series　of　reactions　involving　oxygen　during　sintering.