Abstract: The magnetic particles of Fe3O4 covered with oleic acid were prepared by common chemical coprecipitation method.Covered by the second kind of surfactant,the particles were dispersed in PAO synthetic-oil which has excellent high temperature resistance.In this way,the stable Fe3O4 based magnetic fluid was obtained.The TEM investigation showed that the shape of the particles was nearly spherical and the mean particle size was about 10 nm.The changes of density,viscosity,saturation magnetization and volatilizing rate of the magnetic fluid sample were tested after the sample was heat treated at 170 ℃ for 20 d and the results showed that the density increased slowly,viscosity increased rapidly,the change of saturation magnetization was 0.4%,and the volatilizing rate decreased gradually.