In　order　to　reveal　the　corelations　of　A/O　ratio　and　sludge　settleability,　the　effect　of　different　volume　ratio　(2/6,　4/4,　6/2)　on　activated　sludge　settleability　and　community　of　filamentous　fungus　was　investigated　with　domestic　wastewater　as　influent　during　long-term　operation.　The　resuls　showed　that　good　settleability　of　system　was　maintained　when　A/O　ratio　was　2/6,　and　the　dominant　filamentous　bacteria　was　Type　0041.　When　A/O　ratio　was　increased　to　4/4　the　system　occurred　bulking　sludge　with　SVI　up　to　357　mL/g,　and　the　dominant　filamentous　bacteria　were　Type　0041,　Type　1701.　When　A/O　ratio　of　6/2　was　adopted,　the　system　occurred　slightly　bulking　sludge　and　realised　stable　shortcut　nitrification　and　denitrification.　So　different　A/O　ratio　had　great　influence　on　activated　sludge　settleability,　at　the　same　time　the　intracellular　and　extracellular　storage　characteristics　of　activated　sludge　and　denitrification　and　phosphorus　removal　performance　of　the　system　were　also　severely　affected.　©　2016,　Chinese　Society　for　Environmental　Sciences.　All　right　reserved.