Magnetic iron oxide (Fe3O4) was prepared by a coprecipitation method. Core-shell composite magnetic polymer microspheres with carboxyl groups were synthesized by the dispersion polymerization of styrene and acrylic acid in the presence of magnetic oxide, and dibenzovl peroxide was used as an initiator. The synthesized magnetic polymer microspheres were characterized Nvith X-ray diffraction, transmission electron microscopy, scanning electron microscopy, Fourier transform infrared spectroscopy, and so forth. The results indicated that the product was single-phase Fe3O4 and its average size was about 10 nm. The confi-uration of the microspheres, which contained carboxyl groups, was spherical, and the average size was about 2 pm. The results of vibrating sample magnetometry tests showed that the magnetic powders produced by different surfactants had different saturation magnetizations. When poly(ethylene glycol) with a weight-average molecular weight of 4000 was used as a surfactant, the saturation magnetization of the samples reached 69.2 emu/g. The factors that affected the shape, magnetism, size, and distribution of the microspheres were also studied. (c) 2007 Wiley Periodicals, Inc.