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.