Dissolved　oxygen　(DO)　concentration　is　regarded　as　one　of　the　crucial　factors　to　influence　partial　nitrification　process.　However,　achieving　and　keeping　stable　partial　nitrification　under　different　DO　concentrations　were　widely　reported.　The　mechanism　of　DO　concentration　influencing　partial　nitrification　is　still　unclear.　Therefore,　in　this　study　two　same　sequencing　batch　reactors　(SBRs)　cultivated　same　seeding　sludge　were　built　up　with　real-time　control　strategy.　Different　DO　concentrations　were　controlled　in　SBRs　to　explore　the　effect　of　DO　concentration　on　the　long-term　stability　of　partial　nitrification　process　at　room　temperature.　It　was　discovered　that　ammonium　oxidation　rate　(AOR)　was　inhibited　when　DO　concentration　decreased　from　2.5　to　0.5　mg/L.　The　abundance　of　Nitrospira　increased　from　10(11.5)　to　10(13.7)　copies/g　DNA,　and　its　relative　percentage　increased　from　0.056%　to　3.2%　during　190　operational　cycles,　causing　partial　nitrification　gradually　turning　into　complete　nitrification　process.　However,　when　DO　was　2.5　mg/L　the　abundance　of　Nitrospira　was　stable　and　AOB　was　always　kept　at　10(10.7)　copies/g　DNA.　High　AOR　was　maintained,　and　stable　partial　nitrification　process　was　kept.　Ammonia　oxidizing　bacteria　(AOB)　activity　was　significantly　higher　than　nitrite　oxidizing　bacteria　(NOB)　activity　at　DO　of　2.5　mg/L,　which　was　crucial　to　maintain　excellent　nitrite　accumulation　performance.　(C)　2020　The　Research　Center　for　Eco-Environmental　Sciences,　Chinese　Academy　of　Sciences.　Published　by　Elsevier　B.V.