In　this　paper,　the　performance　of　a　Roots　pump　for　hydrogen　recirculation　in　proton　exchange　membrane　(PEM)　fuel　cell　system　is　simulated　based　on　CFD　modeling.　The　Roots　pump　is　in　a　three-lobe　configuration　with　helical　rotors,　and　it　is　developed　specifically　for　fuel　cell　systems　between　60　to　110　kW.　A　three-dimensional　model　of　the　Roots　pump　is　established　to　predict　the　pump　performance,　including　the　flow　rate　and　power　consumption　under　various　operating　conditions.　Extensive　simulations　were　conducted　and　then　verified　experimentally　by　operating　with　working　fluids　of　air　and　helium.　Based　on　the　validated　CFD　model,　the　contents　of　water　vapor　and　nitrogen　in　the　hydrogen　recirculated　are　taken　into　account　to　evaluate　the　Roots　pump　performance　numerically　according　to　the　actual　conditions　of　the　recirculating　hydrogen　at　the　stack　outlet.　It　is　shown　that　the　volumetric　efficiency　and　isentropic　efficiency　are　improved　with　the　increase　fraction　of　water　vapor　and　nitrogen.　It　is　found　that　the　performance　of　the　Roots　pump　integrated　in　the　PEM　fuel　cell　system　is　between　the　performance　of　the　pump　working　with　air　and　helium.　Finally,　correlations　of　volumetric　efficiency　and　isentropic　efficiency　are　given　based　on　the　CFD　results　to　show　the　general　pattern　of　this　kind　of　hydrogen　pump.　It　is　believed　that　these　equations　are　very　helpful　to　the　design　and　operation　control　of　the　PEM　fuel　cell　system.