We herein report a novel and facile approach to the fabrication of the superparamagnetic magnetite/poly(styrene-co-12-acryloxy-9-octadecenoic acid) nanocomposite microspheres with controllable structure via gamma-ray radiation induced inverse emulsion polymerization under room temperature and at ambient pressure. 12-Acryloxy-9-octadecenoic acid (AOA, containing part of sodium salts Na-AOA) as a surfactant can also copolymerize with the styrene. It is interesting that just by changing the added amount of styrene, the magnetic hollow spheres with different wall thickness and various sizes of core, up to the magnetic solid spheres, can be obtained. The final products were thoroughly characterized by X-ray powder diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), transmission electron diffraction (TEM), field-emission scanning electron microscopy (FESEM), thermogravimetric analysis (TGA) which showed the formation of magnetite/poly(styrene-co-AOA) nanocomposite microspheres. Magnetic hysteresis loop measurements showed that the magnetic nanocomposite microspheres exhibited superparamagnetism, which should make them have potential applications in biotechnology and biomedicine. Furthermore, we also proposed a possible formation mechanism of these magnetic microspheres with different morphologies. (C) 2009 Elsevier Inc. All rights reserved.