In this paper, a free radical polymerization system consisting of DPE was used to prepare magnetic composite microspheres. Fe3O4/P(AA-MMA-St) core-she 1 magnetic composite microspheres have been synthesized by copolymerization of acrylic acid, methyl methacrylate and styrene using DPE as radical control agent in the presence of Fe3O4 nanoparticles. The structure and properties of the magnetic composite microspheres were analyzed by FTIR, H-1 NMR, SEC-MALLS, TEM, TGA, VSM and other instruments, and the formation mechanism of composite microspheres was supposed by those results. It was found that the Fe3O4/P(AA-MMA-St) microspheres were nano-size with relatively homogeneous particle size distribution, perfect sphere-shaped morphologies, superparamagnetism with a saturation magnetization of 18.430 emu/g, and high magnetic content with a value of 40%. H-1 NMR and TEM analysis indicated that at the first stage of polymerization, a DPE-containing copolymer of acrylic acid, methyl methacrylate formed and was then absorbed on the surface of Fe3O4 nanoparticles. Contact angle analysis indicated that the DPE-containing copolymer improved hydrophobicity of Fe3O4 nanoparticles through chemical absorption. In the second step polymerization, certain amount of monomers of styrene and residue methacrylate were initiated by the DPE-containing copolymer on the Fe3O4 nanoparticles' surface and resulted in the formation of Fe3O4/P(AA-MMA-St) composite microspheres. (c) 2009 Elsevier Ltd. All rights reserved.