Abstract: Fe3O4 nanoparticle was prepared by the micro-wave hydrothermal method in the paper. The influences of different factors on the average diameter of Fe3O4 nanoparticle were evaluated by the orthogonal-design method to obtain the optimum parameters for preparing the nanoparticle. Four factors with three levels were considered in the experiment，which were molar proportion of [Fe3+]/[Fe2+]，crystallization temperature，crystallization time and pH. The surface of the Fe3O4 magnetic nanoparticle was modified by oleic acid. The structure，morphology and magnetism of the composite were characterized by X-ray diffraction (XRD)，Fourier transform infrared spectroscopy (FTIR)，transmission electron microscope (TEM) and vibration showpiece magnetor (VSM). The results showed that the coated Fe3O4nanoparticles were uniform spheres and had good dispersion，and its average diameter was about 8 nm. In addition，the composite had superparamagnetism and the saturation magnetization intensity was 61.8 emu/g.