Well-defined monodisperse Fe3O4@poly (2-hydroxyethyl methacrylate)-graft-poly(E-caprolactone) (Fe3O4@PHEMA-g-PCL) magnetic nanoparticles with novel topological structure, i.e., with branched brush polymeric shell, were successfully prepared by the combination of atom transfer radical polymerization (ATRP) and ring-opening polymerization (ROP). Oleic acid stabilized monodisperse Fe3O4 nanoparticles were prepared by a convenient organic phase process and underwent a ligand exchange process with 2-bromo-2-methylpropionic acid (Br-MPA) to generate macroinitiator (Fe3O4@Br-MPA) for ATRP of 2-hydroxyethyl methacrylate (HEMA) to produce Fe3O4@poly(2-hydroxyethyl methacrylate) (Fe3O4@PHEMA). PCL segments were grafted from the side of PHEMA by the ROP of E-caprolactone (CL) with the hydroxyl groups of PHEMA segments used as initiation centers, and then Fe3O4@PHEMA-g-PCL magnetic nanoparticles were obtained. PCL segments of Fe3O4@PHEMA-g-PCL possessed lower degree of crystallinity than that of linear PCL Meanwhile, Fe3O4@PHEMA-g-PCL nanoparticles showed super-paramagnetism and comparatively strong magnetization. In vitro degradation investigation indicated that the degradation rate of PCL segments in Fe3O4@PHEMA-g-PCL increased with the decrease of the length of PCL chains. The release behavior of model drug chlorambucil from the nanoparticles indicated that the rate of drug release could be adjusted by altering the chain-length of PCL segments. (C) 2010 Elsevier Ltd. All rights reserved.