The　Performance　of　the　Anammox　reactor　can　be　improved　by　enhancing　mass　transfer.　Gas　circulation　was　one　of　the　methods.　In　this　study,　the　influence　of　gas　circulation　on　the　performance　of　two　Anammox　reactors　were　investigated.　One　was　floc　sludge　UASB　system,　the　other　was　granular　sludge　UASB　system.　Obtained　results　showed　that　the　removal　efficiency　of　the　floc　sludge　UASB　couldn＇t　be　improved　by　using　the　gas　circulation.　And　serious　loss　of　sludge　was　also　found.　While　in　the　granular　UASB,　the　mixing　characteristics　of　fluid　was　enhanced　and　the　removal　efficiency　was　also　improved　significantly　by　using　the　gas　circulation.　Furthermore,　the　gas　circulation　didn＇t　cause　washout　of　anammox　granules　and　the　deterioration　of　anammox　process.　In　order　to　enhance　the　sludge　settle　ability　as　gas　circulating,　the　structure　of　three-phase　separator　of　the　UASB　need　be　improved　to　enhance　the　gas-sludge　separation.　©　2011　IEEE.