The　SVI,　sludge　bulking　type,　SOUR,　the　denitrification　and　dephosphorization　efficiency,　SND　rate　of　activated　sludge　system　were　investigated　in　a　sequencing　batch　reactor　(SBR)at　low　DO　loading　(-1),　pH　7.2-8.0,　temperature　(23±0.5)°C,　MLSS　3000-3500　mg·L-1.　The　results　showed　that　at　low　DO　levels,　the　bulking　of　sludge　is　not　observed　at　load　of　0.22　g　COD·(g　MLSS)-1·d-1,　while　the　deterioration　of　SVI　accelerates　and　type　of　sludge　bulking　changes　with　the　increase　of　sludge　loads.　At　0.66　g　COD·(g　MLSS)-1·d-1　loading,　non-filamentous　bulking　of　activated　sludge　is　observed　and　the　denitrification　and　dephosphorization　efficiency　goes　down,　but　at　0.44　g　COD·(g　MLSS)-1·d-1　filamentous　bulking　takes　place,　the　denitrification　and　dephosphorization　efficiency　keeps　well,　the　rate　of　phosphorus　absorption　is　faster,　and　SS　in　effluent　is　almost　negligible.　The　SND　rate　is　the　maximum　at　no　bulking　of　activated　sludge,　medium　at　having　non-filamentous　bulking,　and　minimum　at　filamentous　bulking.　If　SOUR　was　over-high　it　is　very　possible　to　droop　sludge　settleability　and　to　deteriorate　water　quality　in　effluent.　©　All　Rights　Reserved.