The　bacterial　composition　of　the　aerobic　granular　sludge　under　filamentous　bulking　was　studied　by　construction　of　16S　rDNA　clone　library.　The　results　indicated　that　the　bacterial　community　in　aerobic　granular　sludge　included　6　major　groups:　β-proteobacteria,　Sphingobacteria,　δ-Proteobacteria,　Flavobacteria,　γ-proteobacteria　and　Actinobacteria,　and　their　portions　were　36.23%,　20.29%,　13.04%,　10.12%,　1.45%　and　1.45%,　respectively.　Sequence　analysis　also　indicated　that　filamentous　bacterial　such　as　Sphaerotilus　natans　existed　in　aerobic　granular　sludge,　but　it　was　not　the　main　cause　of　filamentous　bulking　based　on　the　occurrence　frequency　of　clones　in　16S　rDNA　clone　library.　Filamentous　microorganism　from　outer　edge　of　aerobic　granular　sludge　was　picked　out　for　further　identification　and　was　identified　to　belong　to　filamentous　fungi.　Experiment　proves　that　pH　decreasing　with　running　time　is　the　main　cause　for　filamentous　bulking.　Shorten　running　time　of　the　cycle,　adding　NaHCO3　to　reactor　and　decreasing　aeration　rate　during　process　of　operation　all　can　prevent　filamentous　bulking.　And　as　the　most　direct　and　effective　way,　adding　NaHCO3　to　reactor　even　can　control　filamentous　bulking　at　it　initial　stage.