Carboxyl-functionalized magnetic nanogel was synthesized by facile "green" photochemical method. A possible mechanism of photochemical synthesis was proposed. Effects of irradiation time and volume of monomer dropped on the hydrodynamic diameter of the magnetic nanogel were investigated by photo correlation spectroscopy. The image of aton-Lic force microscopy presented that the magnetic nanogel was with loosed structure. X-ray diffraction analysis showed that UV irradiation did not induce phase change of Fe3O4. Superparamagnetic behaviors were retained for Fe3O4 while slightly reducing the value of saturation magnetization for surface coating. Fligh magnetic content of (as high as 85%) and strong magnetization of Fe3O4 guaranteed that the magnetic nanogel was susceptive to external applied magnetic field. (c) 2007 Wiley Periodicals, Inc.