Fe(3)o4 magnetic nanoparticles (MNPs) were prepared by the co-precipitation of Fe3+ and Fe2+ using NH3 center dot H2O under microwave irradiation to enhance the crystallization of Fe3O4 MNPs. The MNPs were formed in the aqueous phase, modified by oleic acid and migrated to the organic phase of kerosene to form a magnetic fluid (MF). The core size of MNPs was found to be around 10 nm by TEM. FT-IR results indicated that a covalent bond was formed between the hydroxyl groups on the surface of MNPs and the carboxyl groups of oleic acid. The MF demonstrated good stability, and had susceptibility of 7.78 x 10(-4) and saturation magnetization of 27.3 emu/g. In addition, the mean size of aggregates in MF was 27.51 nm. Finally, the rheological property of the prepared MF was investigated using a rotating rheometer attached with a custom-built solenoid coil. It was found that the MF demonstrated shear-thinning behavior and could be described by the Herschel-Bulkley model. (C) 2008 Elsevier B.V. All rights reserved.