Multi-arm star polystyrenes with hyper-branched polyester (HP3) core were prepared by atom transfer radical polymerization (ATRP). The structures of the polymers were investigated with FTIR and H-1 NMR. GPC results showed that the resultant polymers had relatively broad polydispersity indices that arouse from the macromolecular initiator (HP3-Br). The thermal properties were studied using differential scanning calorimetry (DSC) and thermogravirnetric analysis (TGA). DSC analysis indicated that polystyrene star polymers had only the glass transition temperatures (T-g), which changes with the weight ratio of multifunctional macroinitiator-to-monomer. In addition, these star polymers could form the spherical micelles in the selected solvent (THF/n-hexane). (c) 2005 Wiley Periodicals, Inc.