A biodegradable block copolymer, POIY-D,L-lactide (PLA)-co-poly(ethylene glycol) (PEG), was prepared by the ring-opening polymerization of lactide with stannous caprylate [Sn(OCt(2))] as a catalyst; then, the PLA-PEG copolymer was made into nanoparticles by nanoprecipitation under different conditions. The average molecular weight and structure of PLA-PEG were detected by H-1-NMR and gel permeation chromatography. The sizes and distributions of the nanoparticles were investigated with a laser particle-size analyzer. The morphologies of the nanoparticles were examined by transmission electron microscopy. The effects of the solvent-nonsolvent system, operation conditions, and dosage of span-80 on the sizes and distributions of the nanoparticles are discussed. The results show that acetone-water was a suitable solvent-nonsolvent system and the volume ratio of the nonsolvent phase to the solvent phase (O/W) (v/v), the concentration of PLA-PEG in the solvent phase, and the dosage of span-80 had important effects on the particle sizes and distributions. (c) 2005 Wiley Periodicals, Inc.