The　nitrogen　removal　performance　of　anammox　(anaerobic　ammonium　oxidation)　granules　in　different　sizes　R1(above　2.5　mm),　R2(1.5~2.5　mm),　and　R3(0.5~1.5　mm)　were　investigated　by　serum　bottle　batch　tests　under　25.　The　specific　anammox　activities　of　granules　R1,　R2,　R3　were　0.555,　0.423,　0.456kgN/(kgVSS·d),　respectively.　The　value　of　R1was　the　highest,　while　the　values　of　R2　and　R3　were　similar.　The　average　specific　anammox　activity,　denitrification　activity　of　nitrite,　denitrification　activity　of　nitrate,　aerobic　ammonium　oxidation　activity　and　aerobic　nitrite　oxidation　activity　were　0.478,　0.028,　0.037,　0.050,　0.033　kgN/(kgVSS·d),　respectively.　The　SEM　indicated　that　some　holes　lied　in　the　outer　part　of　anammox　granule,　which　redused　the　adverse　effects　of　particle　size　on　mass　transfer　efficiency.　In　the　outer　part　of　anammox　granule,　bacteria　were　mainly　spherical,　which　may　be　AOB.　In　the　inner　part　of　the　anammox　granule,　the　bacteria　were　mainly　crater-shaped,　which　should　be　anammox　bacteria.　Owing　to　this　structure,　the　anammox　bacterium　was　able　to　relieve　from　the　adverse　effects　of　oxygen.　©,　2014,　Chinese　Society　for　Environmental　Sciences.　All　right　reserved.