Fe3O4 magnetic nanoparticles were prepared by coprecipitation using NH3 center dot H2O as the precipitating agent, and were characterized by Fourier transform infrared spectroscopy (FTIR), transmission electron microscopy (TEM), and X-ray powder diffraction (XRD). The compatibility between the Fe3O4 nanoparticles and water were enhanced by grafting acrylic acid onto the nanoparticle surface. FTIR, XRD, thermogravimetry (TG), and differential scanning calorimetry (DSC) were used to characterize the resultant sample. The effects of initiator dosage, monomer concentration, and reaction temperature on the characteristics of surface-modified Fe3O4 nanoparticles were investigated. Moreover, magnetic fluids (MF), prepared by dispersing the PAA grafted Fe3O4 nanoparticles in water, were characterized using UV-vis spectrophotometer, Gouy magnetic balance, and laser particle-size analyzer. The rheological characteristics of magnetic fluid were investigated using capillary and rotating rheometers. The MF was added to prepare PVA thin film to improve mechanical properties. (C) 2007 Wiley Periodicals, Inc.