As　a　smart　material　with　special　rheological　properties,　the　magnetorheological　(MR)　fluid　can　be　used　to　produce　ideal　semi-active　control　devices,　i.e.　MR　dampers.　In　consideration　of　the　limitations　of　current　MR　damper,　a　reversible　MR　damper　(RMR　damper)　is　presented　in　this　paper.　For　the　new　type　damper,　both　permanent　magnets　and　magnetic　coils　are　installed.　Hence,　the　RMR　damper　can　operate　with　maximal　damping　force　once　power　fails,　and　the　damping　force　can　also　be　adjusted　to　the　demand　value　with　power　supply　so　as　to　raise　the　reliability　and　practicability　in　earthquake.　The　magnetic　circuit　of　RMR　damper　is　developed,　and　the　design　theory　is　described.　A　full-scale　RMR　damper　with　a　bypass　valve　is　manufactured,　and　several　reliability　tests　are　presented.　The　results　demonstrate　that　the　magnetic　circuit　can　change　the　magnetic　flux　density　stably　and　reliably　under　the　applied　current　and　the　performance　is　excellent.　Based　on　the　results,　some　improvement　is　presented　to　the　damper.　Copyright　(C)　2009　John　Wiley　&　Sons,　Ltd.