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(SrFe_(12)O_(19))nanoparticles as an additive.The sedimentation stability of MRFs was studied.The results indicated that the stability of MRFs was improved remarkably by adding SrFe_(12)O_(19) nanoparticles and the sedimentation ratio was only 0.88 in 20 days when the content of nanoparticles reached 10 wt%.The Theological properties were characterized by a HAAKE rheometer without a magnetic field and a capillary rheometer with and without a magnetic field.The effects of SrFe_(12)O_(19) nanoparticles,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. 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 (SrFe 12 O 19) nanoparticles as an additive.The sedimentation stability of MRFs was studied. The results indicated that the stability of MRFs was improved remarkably by adding SrFe_ (12) O_ (19) nanoparticles and the sedimentation ratio was only 0.88 in 20 days when the content of nanoparticles reached 10 wt%. Theological properties were characterized by a HAAKE rheometer without a magnetic field and a capillary rheometer with and without a magnetic field. These effects of SrFe 12 O 19 nanoparticles, 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 appli ed magnetic field.