Seawater　reverse　osmosis　(SWRO)　develops　rapidly　and　benefits　from　excellent　performance　of　isobaric　energy　recovery　devices　(ERDs).　Leakage　characteristics　directly　affect　the　ERD　performance.　In　this　paper,　leakage　characteristics　of　a　fully-rotary　valve　energy　recovery　device　(FRV-ERD)　which　is　a　new　type　of　isobaric　ERD　are　studied　numerically.　Leakage　variation　of　the　fully-rotary　valve　(FRV)　that　is　the　key　part　of　the　device　caused　by　pressure　difference　and　geometric　parameters　is　investigated.　Then　performance　of　the　FRV-ERD　is　discussed.　Results　indicate　that　leakage　rates　of　forward　and　reverse　leakage　within　the　same　FRV　are　almost　equal　to　each　other　and　present　a　good　linear　relationship　with　pressure　difference.　Leakage　rate　is　exactly　proportional　to　the　3.0　power　of　the　clearance　height.　Effects　caused　by　changing　the　length　and　diameter　are　equivalent.　The　leakage　rate　decreases　sharply　first　and　then　slowly　after　the　length　or　diameter　is　larger　than　a　specific　value　of　which　the　axial　length　and　the　half　circumference　are　equal　to　each　other.　The　performance　of　the　larger　size　FRV-ERD　is　much　better　than　the　smaller　one,　and　the　required　clearance　is　also　significantly　increased,　alleviating　machining　difficulty　to　some　extent.