SBR　reactor　was　performed　to　incubate　polyphosphate　accumulating　organism　(PAO),　and　it　was　checked　out　of　the　system　by　fluorescence　in　situ　hybridization.　As　PAO　is　a　kind　of　ordinary　heterotrophic　bacteria,　it　was　excluded　the　ability　of　phosphate　release　and　uptake　and　it　was　considered　only　the　capacity　of　denitrification　of　the　target　biomass.　The　results　indicated　that　acetate　and　PHB　can　be　the　electron　donors　of　PAO　to　denitrify.　When　fed　with　acetate,　the　denitrifying　rate　and　PHB　producing　rate　were　independent　of　initial　nitrate　concentration.　However,　served　as　more　nitrate　in　the　reactor,　it　would　be　less　PHB　produced　and　fewer　nitrate　reducing　when　using　same　amount　of　acetate.　In　view　of　PHB　stored　as　an　internal　carbon　and　energy　source,　it　presented　as　a　reaction　of　zero-order　to　the　substrate　by　PAO　to　denitrify,　such　as　nitrate,　besides,　the　specific　denitrifying　rate　was　0.9733　mg/(g　x　h)　and　the　specific　PHB　consuming　rate　was　2.4626　mg/(g　x　h).