Ferrofluids (FFs), prepared from Fe3O4 ferromagnetic nanoparticles (FMNPs) by agitation and ball milling, respectively, were characterized using a capillary rheometer and a rotating rheometer. The effects of surfactant dosage and temperature on the viscosity of dilute FFs were measured using the capillary rheometer. The rheological properties of high-concentration FFs, prepared using a high-energy ball mill were measured using a rotating rheometer. The yield stress under applied magnetic field was also obtained using rotating and capillary rheometers. Constitutive equations of FFs with/without applied magnetic field were correlated using the Herschel-Bulkley (H-B) model. Finally, a general constitutive equation of the shear stress was proposed, and a theoretical model for the yield stress threshold under applied magnetic field was derived and was compared with experimental measurements. (C) 2007 Published by Elsevier Ltd.