Abstract: The effect of washing by ethanol for different times on the surface structure of oleic acid-coated Fe₃O₄ nanoparticles was studied.The FT-IR and TGA were used together to study the evolution of surface structure of oleic acid-coated Fe₃O₄ nanomagnetic particles.TEM were used to study the morphology of the Fe₃O₄ magnetic nanoparticles after coated.The results showed that after five times washing,the bilayer oleic acid-coated structure was attained,while after twenty times washing,the physically absorbed secondary layer was washed away by the ethanol and the bilayer oleic acid-coated structure evolved into the single layer.