Atom transfer radical emulsion polymerization of styrene using PEG-Cl as macroinitiator under microwave irradiation was successfully conducted and monodispersed nanoparticles were prepared. The PEG-Cl macroinitiator was synthesized, and confirmed by FTIR spectrum. The structure of the PEG-b-PSt diblock copolymer was characterized by H-1-NMR and the number of styrene unit in the diblock copolymer was calculated. The morphology, size, and size distribution of the nanoparticles were characterized by transmission electron microscope (TEM) and photon correlation spectroscopy (PCS). The effects of the ratio of macroinitiator and monomer, the ratio of catalyst and macroinitiator on the size and size distribution of nanoparticles were investigated. It was found that the diameters of PEG-b-PSt nanoparticles prepared under microwave irradiation were smaller (<50 nm) and more monodispersed than those prepared with conventional heating. Moreover, with the increasing of the ratio of St/PEG-Cl, the hydrodynamic diameters (D-h) of the nanoparticles increased and the poly index decreased, both Dh and poly index of the nanoparticles prepared under microwave irradiation were smaller then those prepared with conventional heating; as the concentration of catalyst increased, the Dh of the nanoparticles decreased and the poly index of the nanoparticles increased. (C) 2007 Wiley Periodicals, Inc.