Superparamagnetic iron oxide nanoparticles were prepared in the presence of poly (ethylene oxide)graft-poly (acrylic acid) copolymers via an in situ templated coprecipitation process. The resulted composite nanoparticles consisted of clusters of iron oxide monocrystals embedded inside the polymer chains, and were highly dispersible in aqueous solution. hi these graft copolymers, the PAA segments were chemisorbed onto the Fe3O4 nanoparticle surface through their carboxylic acid groups by forming carboxylate groups with the Fe atoms, while PEO segments as the stabilizers extended into the water matrix. Thus the composite nanoparticles exhibited narrow size distribution by the dynamic light scattering measurement. X-ray diffraction and magnetometer data confirmed the crystalline structure and superparamagnetic property of the particles. This procedure provided a good choice for preparing stable iron oxide magnetic nanoparticles with PEO modified surfaces. (c) 2008 Wiley Periodicals, Inc.