Hollow　anammox　granules　have　been　concerned　in　the　field　of　anammox　granular　sludge　technology.　So　far,　little　information　is　available　about　the　effect　of　shear　stress　on　hollow　anammox　granules.　The　evolution　of　hollow　granular　sludge　at　three　shear　stresses　(τ　=　0.014,　0.041,　and　0.075　Pa)　was　investigated　in　three　laboratory　scale　continuous　stirred　tank　reactors.　It　was　found　that　the　lower　shear　stress　(0.014　Pa)　was　beneficial　to　the　retention　of　granules　and　thus　ensured　a　higher　nitrogen　removal　rate　(NRR).　A　higher　shear　stress　led　to　a　stronger　granular　sludge　abrasion　and　thus　smaller　granule　size.　The　results　showed　that　the　shear　stress　affected　the　production　of　extracellular　polymers　(EPS).　A　higher　shear　stress　resulted　in　a　higher　content　of　EPS.　Then,　the　shear　stress　had　a　physical　impact　on　the　EPS,　which　in　turn　affected　the　physical　and　chemical　properties　of　the　particles.　The　increasing　shear　stress　could　increase　the　loose　degree　of　loosely-bound　EPS　(LB-EPS),　and　thus　facilitated　mass　transfer,　and　then　enhanced　the　permeability　of　anammox　granules.　On　the　contrary,　the　lower　shear　stress　not　only　weakened　the　penetration　of　the　substrate,　but　also　hindered　the　outflow　of　gas　to　the　outside　of　the　granules.　Therefore,　the　occurrence　of　cavity　changed　the　internal　structure　of　the　anammox　granules　and　affected　the　distribution　of　bacteria.　In　conclusion,　this　research　clarified　the　impact　mechanism　of　shear　stress　on　the　characteristics　of　hollow　anammox　granules　and　provided　theoretical　guidance　for　the　application　in　engineering.　©　2022　Elsevier　Ltd.