Abstract: Superparamagnetic Fe3O4 nanoparticles were first synthesized by chemical co-precipitation method. Then, the surfaces of Fe3O4 nanoparticles were successively modified with oleic acid and sodium oleate to obtain stable magnetic fluid in water. Finally, Fe3O4-poly(methyl methacrylate) (PMMA) composite nanospheres bearing carboxylic functional group on the surface, were faciely prepared by one-step emulsion polymerization in the presence of the magnetic fluid. Dynamic light scattering, transmission electron microscopy, Fourier transform infrared spectroscopy, thermogravimetry and vibrating sample magnetometer measurements were used to characterize the size, morphology, structure, composition and magnetic property of the composite nanospheres, respectively. Results show that the obtained composite nanospheres were of an average diameter of about 122 nm and with carboxylic functional groups on their surfaces. Also, they were superparamagnetic and with a fairly high saturation magnetization at room temperature.