Abstract　The　heat　transfer　oil-based　magnetorheological　fluid　(MRF)　was　prepared　using　oleic　acid-modified　micron　carbonyl　iron　powder　as　a　magnetic　dispersed　phase　and　strontium　hexaferrite　(SrFe12O19)　nanoparticles　as　an　additive.　The　sedimentation　stability　of　MRFs　was　studied.　The　results　indicated　that　the　stability　of　MRFs　was　improved　remarkably　by　adding　SrFe12O19nanoparticles　and　the　sedimentation　ratio　was　only　0.88　in　20　days　when　the　content　of　nanoparticles　reached　10　wt%.　The　rheological　properties　were　characterized　by　a　HAAKE　rheometer　without　a　magnetic　field　and　a　capillary　rheometer　with　and　without　a　magnetic　field.　The　effects　of　SrFe12O19nanoparticles,　the　temperature,　and　magnetic　field　strength　were　investigated.　In　addition,　the　rheological　properties　could　be　predicted　well　using　the　improved　Herschel-Bulkley　model,　even　under　a　magnetic　field.　A　theoretical　model　was　also　proposed　to　predict　the　yield　stress　based　on　the　microstructure　of　the　MRF　under　an　applied　magnetic　field.　©　2014　Chinese　Society　of　Particuology　and　Institute　of　Process　Engineering,　Chinese　Academy　of　Sciences.