Abstract: Magnetic fluid containing Fe₃O₄ as mother nucleus was prepared by chemical coprecipitation and treated with dextran-70 for surface modification. The dextran-70-modified magnetic fluid was reacted with 6-bromohexanoic acid and their chemical stability was studied, with the structure characterized by using laser granulometer, X-ray diffraction, Fourier-transform infrared analysis and transmission electron microscopy. The results showed that the magnetic fluid with surface modification by dextran-70 exhibited high stability in the presence strong alkali or acid. No chemical changes occurred in the magnetic Fe₃O₄ nucleus surrounded by alkali-treated dextran-70 and the organic chain linked to the surface dextran rendered the reaction to the anti-cancer drug such as mitomycin C possible, suggesting the potential of the preparation as a drug carrier.