Anaerobic　ammonium　oxidation　(Anammox)　is　acknowledged　as　one　of　the　most　cost-efficient　nitrogen　removal　biotechnologies　from　wastewater.　Anammox　bacteria　(AnAOB)　tend　to　self-aggregate　to　form　the　granules,　which　can　harbor　symbiotic　heterotrophic　denitrifying　bacteria　(HDB).　However,　the　dynamics　of　microbial　synergism　between　AnAOB　and　HDB　in　size-fractioned　anammox　granules　are　still　unclear.　This　study　systematically　investigated　the　bioactivity　and　bioabundance　of　highly　enriched　anammox　granules　with　different　sizes　by　combining　specific　activity　tests,　DNA　sequencing　and　bioprocess　modeling.　The　medium　granules　of　1.0-2.0　mm　were　the　most　distributed　size　in　the　whole　reactor　with　the　volumetric　percentage　of　55.7　+/-　7.5%.　The　medium　-large　granules　held　the　highest　total　nitrogen　removal　rate　of　1.10　+/-　0.05　mmol　N2/(g　VSS　&　sdot;h),　where　AnAOB　and　HDB　co-existed　with　anammox　contributing　93.1　+/-　0.5%　to　N2　production.　Candidatus　Kuenenia,　Denitratisoma　and　Anaerolineaceae　were　the　only　three　genera　that　could　be　highly　detected,　dominating　52.5-65.6%　of　mi-crobial　communities.　Kuenenia　(AnAOB)　only　held　the　positive　correlation　with　Denitratisoma　(HDB)　by　co　-occurrence　network　analysis.　Furthermore,　AnAOB　were　proved　to　have　a　synergistic　growth　with　HDB,　form　the　nitrite　cycle,　and　maximize　at　the　medium　size　by　joint　analysis　of　denitrifying　functional　gene　and　steady-state　simulation.　Collectively,　we　proposed　that　the　medium-sized　anammox　granules　of　1.0-2.0　mm　could　reach　the　optimal　synergism　between　AnAOB　and　HDB　via　nitrite　cycle.　This　work　could　provide　insights　into　the　microbial　synergism　in　anammox　granules　and　benefit　the　development　of　size-control　strategy　for　anammox　operation.