Magnetite (Fe3O4) nanoparticles were synthesized by chemical precipitation. To reduce the aggregation of Fe3O4 nanoparticles, an effective surface modification method was proposed by grafting polystyrene onto the Fe3O4 particles. The results of Fourier transform infrared spectra and elemental analysis showed that the polymer chains have been successfully grafted from the surface of the Fe3O4 nanoparticles and that the percentage of grafting can reach 73%. Transmission electron microscope showed that grafted polymer chains on nanoparticles could prevent the aggregation of Fe3O4 nanoparticles markedly in toluene and improve their compatibility with organic phase. Another finding was the grafting reaction did not alter the crystalline structure of the Fe3O4 nanoparticles according to the X-ray diffraction patterns, and the saturation magnetization of PS-Fe3O4 nanoparticles was found to be lower than bulk magnetite.