A series of hyperbranched poly(amine-ester)-co-D,L-lactide (HPAE-co-PLA) copolymer were synthesized by ring-opening polymerization of D,L-lactide with Sn(Oct)(2) as catalyst to a fourth generation branched poly(amine-ester) (HPAE-OHs4). The chemical structures of copolymers were determined by FTIR, H-1-NMR, C-13-NMR, and TGA. Double emulsion (DE) and nanoprecipitation (NP) method were used to fabricate the nanoparticles of these copolymers encapsulating bovine serum albumin (BSA) as a model. DSC thermo-grams indicated that the nanoparticles with BSA kept stable below 40 degrees C. Different factors which influence on particular size and encapsulation efficiency (EE) were investigated. Their EE to BSA could reach 97.8% at an available condition. In vitro release behavior of NPs showed a continuous release after a burst release. The stability maintenance of BSA in the nanoparticle release in vitro was also measured via circular dichroism and fluorescence spectrometry. The results showed that the copolymer nanoparticles have a promising potential in protein delivery system. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 117: 1156-1167, 2010