Abstract: Fe3O4 nanoparticles coated by oleate were prepared by means of coprecipitation method. The influence of temperature, concentration, stirring speed and feeding method on particle′s size was studied. Then the particles were coated with dextran for the second layer, and coupled with polypeptide to apply in Magnetic Resonance Imaging(MRI). The results showed that the particles obtained under stirring speed of 7 000 r/min, temperature of 70 ℃ and FeCl2 concentration of 01 mol/L were about 8 nm in size and superparamagetic with singledomain crystal structure. The composite Fe3O4 particles can specifically gather in the tumor with the concentration of 213 times of physiological saline.