A novel preparation method of net magnetic polymer nanogels with swollen shell was developed. UV-induced photopolymerization of 2-hydroxyethyl methacrylate (HEMA) was performed in the magnetite aqueous suspension, free of any additive to obtain monodisperse magnetic nanogels with swollen shell. Owing to the step-growth polymerization of the monomer, the particle size of magnetic nanogels can be conveniently manipulated by changing the irradiation time and the volume of the monomer dropped. The crystalline structure of Fe3O4 core and chemical composition of PHEMA magnetic nanogels were characterized by using X-ray diffraction, Fourier transform infrared spectroscopy, and thermogravimetric analysis, respectively. Particle sizes, size distributions of magnetic nanogels in swollen state and dry state were measured by photo-correlation spectroscopy (PCS) and transmission electron microscopy (TEM), respectively. The morphology of swollen magnetic nanogels and that of polymer capsules obtained from magnetic nanogels etched by hydrochloride acid were observed by TEM. The high magnetizations and superparamagnetic behaviors of naked Fe3O4 and magnetic nanogels at room temperature were confirmed by the measurement of hysteresis curves. (c) 2006 Wiley Periodicals, Inc.