An　integrated　investigation　to　structural,　activity　and　microbial　diversity　of　anammox　granular　sludge　(AnGS)　in　a　wastewater　treatment　system　with　high　ammonia　nitrogen　load　was　performed　and　aimed　to　establish　the　relationship　between　granular　size　and　performance.　With　the　increase　in　granule　size,　the　main　component　of　extracellular　polymeric　substances　(EPS)　changed　from　slime　EPS　to　tightly-bound　EPS,　while　the　organic　component　remained　the　same,　and　the　specific　anammox　activity　increased.　However,　the　results　of　qPCR　and　high-throughput　sequencing　showed　that　for　granules　with　sizes　inferior　than　4.75　mm,　the　abundance　of　ammonia-oxidizing　bacteria　(AnAOB)　increased　as　the　size　increased,　and　the　copies　of　AnAOB　decreased　when　the　granule　size　increased　above　4.75　mm,　and　the　community　complexity　increased.　According　to　the　correlation　analysis　results,　AnAOB　first　accumulated　and　then　optimized　the　flora　structure　to　improve　efficiency　and　2.8　mm　to　4.75　mm　was　the　optimal　size　of　AnGS.