In this paper, oleic acid coated Fe3O4 nanoparticles were functionalized by 3,4-Dihydroxybenzenepropionic acid (DHCA) in tetrahydrofuran (THF). After conjugated by polyethylene (PEI) the functionalized Fe3O4 nanoparticles were well dispersed in water. The morphology, size distribution, magnetic properties and potential of the magnetic particles were characterized by TEM FT-IR, VSM, and Zeta potential, respectively. The results show that this method is simple, the PEI functionalized Fe3O4 nanoparticles have superparamagnetic properties and excellent dispersion.