A novel biodegradable amphiphilic copolymer with hydrophobic poly(C-caprolactone) branches containing cholic acid moiety and a hydrophilic poly(ethylene glycol) chain was synthesized. The copolymer was characterized by FTIR, H-1 NMR, gel permeation chromatography (GPC), differential scanning calorimetry (DSC), polarizing light microscopy (PLM), and wide-angle X-ray diffraction (WAXD) analysis. The amphiphilic copolymer could self-assemble into micelles in an aqueous solution. The critical micelle concentration of the amphiphilic copolymer was determined by fluorescence spectroscopy. A nanoparticle drug delivery system with a regularly spherical shape was prepared with high encapsulation efficiency. The in vitro drug release from the drugloaded polymeric nanoparticles was investigated. Because of the branched structure of the hydrophobic part of the copolymer and the relatively fast degradation rate of the copolymer, an improved release behavior was observed. (c) 2007 Wiley Periodicals, Inc.