The　behavior　of　biological　phosphorus　(P)　and　nitrogen　(N)　removal　in　a　lab-scaled　anaerobic/anoxic/oxic　(A2/O)-biological　aerated　filter　(BAF)　combined　system　were　investigated　during　the　treatment　of　real　domestic　sewage　with　low　C/N　ratio.　In　order　to　solve　the　conflict　of　sludge　retention　time　(SRT)between　nitrifiers　and　polyphosphate-accumulating　organisms(PAOs),　a　short　SRT　was　applied　in　the　A2/O　reactor　to　wash　out　the　nitrifiers.　Therefore,　in　the　system,　the　A2/O　reactor　was　mainly　used　for　P　removal　and　denitrification　and　the　BAF　reactor　was　used　for　nitrification.　The　experimental　results　clearly　showed　that　COD,　N　and　P　can　be　simultaneously　removed　in　this　combined　system.　When　the　C/N　ratio　was　4.2　and　the　internal　recycle　ratio　was　250%,　the　concentrations　of　COD,　total　nitrogen　(TN)　and　total　phosphorus(TP)were　reduced　from　239.9　mg　·　L-1,　57.3　mg　·　L-1　and　5.1　mg　·　L-1　in　the　raw　waste　water　to　34.1　mg　·　L-1,　13.3　mg　·　L-1　and　0.1　mg　·　L-1　in　the　effluent,　respectively.　The　removal　efficiencies　of　COD,　TN　and　TP　reached　to　85.8%,　76.9%　and　98.3%,　respectively.　In　addition,　almost　100%　ammonium　nitrogen　removal　efficiency　was　achieved　in　the　BAF　reactor.　Batch　tests　showed　that　the　population　of　denitrifying　phosphate-accumulating　organisms　(DPAOs)　was　up　to　40.5%　of　the　total　PAOs.