Magnetite (Fe3O4) nanoparticles with the surface hydroxyl groups were achieved by a polyol process. Using 2-(4-chlorosulfonylphenyl) ethyltrichlorosilane (CTCS) as initiator, 2-(methacryloyloxy)ethyl phosphorylcholine (MPC) as monomer, poly(2-(methacryloyloxy)ethyl phosphorylcholine) (PMPC)-grafted Fe3O4 nanoparticles (MNP) were successfully prepared via the atom transfer radical polymerization (ATRP) method. The successful grafting of PMPC on the Fe3O4 nanoparticles surface was ascertained from the FTIR analysis. The modified nanoparticles (MNP-CTCS-PMPC) showed a good biocompatibility in the cytotoxicity test in vitro. Performance testing of MNP-CTCS-PMPC was performed through magnetic resonance analysis (MR), and its r2/r1 value was 24.1. These results indicated that the modified Fe3O4 nanoparticles would be a potential MRI contrast reagent. © 2012 Wiley Periodicals, Inc. J Biomed Mater Res Part A, 2013.