Excessive　electric　field　strength　causes　the　droplets　to　contact　but　not　coalesce,　severely　deteriorating　the　oil-water　separation　efficiency.　The　investigation　of　the　non-coalescence　of　nonidentical　droplets　in　oil　under　a　DC　electric　field　is　conducted　using　high-speed　photography.　The　effects　of　droplet　volume　ratio　and　interfacial　tension　ratio　on　the　characteristics　and　critical　parameters　for　droplet　non-coalescence　are　systematically　explored.　The　results　show　the　critical　electrocapillary　number　for　the　non-coalescence　of　nonidentical　droplets　and　critical　contact　cone　angle　decreases　as　the　volume　sum　increases　or　the　interfacial　tension　ratio　decreases,　which　is　related　to　the　competition　between　the　resultant　electric　field　force　and　the　interfacial　tension　force.　These　results　contribute　to　optimizing　the　operating　parameters　of　the　electric　coalescer　and　improving　the　oil-water　separation　efficiency.