The　effect　of　dissolved　oxygen　(DO)　on　the　lab-scale　step-feed　biological　nitrogen　removal　process　treating　domestic　wastewater　was　investigated.　Different　DO　concentration　in　aerobic　tank　2.8,　1.7,　0.8,　0.5　mg/L　was　obtained　by　controlling　the　aeration　rates,　which　was　0.9,　0.6,　0.4,　0.3　m3/h,　respectively.　Better　nitrification　efficiency　could　be　obtained　with　appropriate　control　under　lower　DO　concentration　of　0.5　mg/L　in　the　aerobic　tank,　the　ammonia　removal　efficiency　was　more　than　98%.　Simultaneously,　due　to　the　decreased　DO　circulated　from　the　aerobic　tank　to　the　anoxic　tank　and　the　simultaneous　nitrification　and　denitrification　(SND)　that　occurred　in　the　aerobic　tank　under　lower　DO　concentration,　the　total　nitrogen　removal　efficiency　under　lower　DO　concentration　was　far　better　than　under　higher　DO　concentration.　In　addition,　the　long-time　stable　operation　of　the　step-feed　biological　nitrogen　removal　process　with　better　sludge　settling　performance　under　lower　DO　concentration　was　attributed　to　the　special　process　configuration.