Shifts　in　bacterial　community　composition　and　abundance　of　nitrifiers　during　aerobic　granulation,　and　the　effects　of　wastewater　composition　on　them　were　investigated　using　Illumina　sequencing　and　quantitative　PCR.　The　bacterial　diversity　decreased　sharply　during　the　post-granulation　period.　Although　cultivated　with　different　wastewater　types,　aerobic　granular　sludge　(AGS)　formed　with　similar　bacterial　structure.　The　bacterial　structure　in　AGS　was　completely　different　from　that　of　seed　sludge.　The　minor　genera　in　seed　sludge,　e.g.,　Arcobacter,　Aeromonas,　Flavobacterium　and　Acinetobacter,　became　the　dominant　genera　in　AGS.　These　genera　have　the　potential　to　secrete　excess　extracellular　polymer　substances.　Whereas,　the　dominant　genera　in　seed　sludge　were　found　in　less　amount　or　even　disappeared　in　AGS.　During　aerobic　granulation,　ammonia-oxidizing　archaea　were　gradually　washed-out.　While,　ammonia-oxidizing　bacteria,　complete　ammonia　oxidizers　and　nitrite-oxidizing　bacteria　were　retained.　Overall,　in　this　study,　the　bacterial　genera　with　low　relative　abundance　in　seed　sludge　are　important　for　aerobic　granulation.