The magnetic properties, infrared absorption spectra and M?ssbauer spectra of dif f irent size magnetite particles were measured before and after coating with organic surfactant. The experimental results show that the nature of coating is chemical absorption between the fine magnetite particle surface and the surfactant. After coating the specific magnetization is reduced under equal magnetic field condition and M?ssbauer spectra become more broadened with the magnetite particle size decreasing. The experimental results may be explained by the surface anisotropy theory.