The　innovative　Partial　Denitrification-hydroxyapatite　(PD-HAP)　coupled　granule　offers　a　promising　biotechnology　for　supplying　nitrite　to　Anammox　while　simultaneously　removing　phosphorus.　This　study　systematically　examined　the　characteristics,　biomass　activity,　and　microbial　community　of　PD-HAP　granules　of　various　sizes　to　enhance　system　stability　and　phosphorus　removal　efficiency.　The　results　showed　that　larger　granules　had　higher　inorganic　mineral　and　extracellular　polymeric　substances　(EPS)　content.　These　factors　contributed　to　their　denser　structure,　smoother　surfaces,　and　better　settling　velocities　compared　to　smaller　granules.　In　contrast,　smaller　granules　exhibited　higher　denitrification　activity　due　to　their　greater　biomass　content　and　more　efficient　mass　transfer.　Calcium　and　phosphorus　were　identified　as　the　primary　inorganic　constituents,　mainly　in　the　form　of　HAP,　and　were　especially　abundant　in　larger　granules.　Granules　larger　than　3.0　mm　demonstrated　a　higher　capacity　for　phosphorus　removal.　This　was　due　to　their　significantly　greater　tightly　bound　EPS　(T-EPS)　content,　which　led　to　increased　HAP　and　phosphorus　levels.　All　granule　sizes　maintained　a　nitrate-to-nitrite　transformation　ratio　(NTR)　above　80　%,　primarily　due　to　the　dominance　of　Flavobacterium　(65.8　%-74.9　%).　Although　larger　granules　had　lower　denitrification　activity,　they　still　achieved　an　excellent　NTR　(above　85　%).　They　also　showed　superior　settling　velocity　(174.9　m/h)　and　stronger　phosphorus　removal　capacity　(54.1　mg/g　SS)　compared　to　smaller　granules.　Maintaining　a　dominant　proportion　of　granules　larger　than　3.0　mm　could　be　an　effective　strategy　to　enhance　both　system　stability　and　phosphorus　removal.　This　study　provides　valuable　insights　into　the　PD-HAP　coupled　granular　process,　promoting　efficient　nitrite　production　and　effective　phosphorus　removal.