Abstract: Nano Fe3O4 particles were successfully prepared by chemical co-precipitation process.The effect of amount of substance ratio of Fe2+ and Fe3+ on diameter and magnetization of Fe3O4 particles was systematically studied.Different surfactants were used to prepare silicon-oil-based magnetic fluid.The results indicat that the amount of substance ratio of Fe2+ and Fe3+ had obvious effect on the properties of Fe3O4 particles,but had little effect on particle size which was all for nanometer.When the amount of substance ratio of Fe2+ and Fe3+ was 0.6,the saturation magnetization reached the maximum,52.1 A·m2·kg-1.Though the dispersion performance of polyvinyl pyrrolidone(PVP) was better than that of oleic acid in ethanol,bad compatibility with silicon oil and tackifying effect limited its application.Silicon-oil-based nano magnetic fluid could be prepared using oleic acid as surfactant because of excellent lipophilicity.