In　order　to　study　the　growth　of　the　bacterium,　the　experiments　were　designed　to　enrich　phosphate　accumulating　organisms　(PAOs)　under　alternate　anaerobic/aerobic　conditions　which　fed　acetate/propionate　in　synthetic　water.　Molecular　biology　techniques　of　FISH　and　PCR-DGGE　were　used　to　qualitatively　and　quantitatively　analyze　bacterial　population　of　EBPR.　DGGE　results　showed　that　the　bacterial　population　had　undergone　significant　changes　which　some　indices,　including　Shannon　index　(H＇),　Margalef　index　(DMg)　and　the　number　of　bands　(R)　had　similar　fluctuations,　and　had　the　highest　value　at　the　end　of　phase　II,　which　had　decreased　in　the　stable　operation　of　the　phase(phase　III).　the　dominance　index(d)　had　increased　in　the　phase　III.　Cluster　dendrogram　analysis　results　showed　that　the　bacterial　population　had　higher　similarity　in　the　phase　III.　FISH　results　showed　that,　PAOs　and　GAOs　had　the　similar　increased　trends　in　the　start-up　phase　and　increase　load　phase,　and　accounted　for　41%　and　39%　of　total　biomass.　PAOs　became　dominance　with　compared　to　GAOs(<6%)　and　accounted　for　89%　of　total　biomass　in　the　phase　III.