Copolymers of epsilon-caprolactone and L-lactide (PCLLA) with different monomer ratio were synthesized by ring opening polymerization, and drug-loaded nanoparticles of poly-epsilon-caprolactone (PCL), poly-L-lactide (PLLA), and their copolymers were prepared by precipitation method, respectively. The results of differential scanning calorimetry and X-ray diffraction indicated that the copolymerization of PCLLA decreased the crystallinity of the polymers, and the results of transmission electron micrograph and laser light scattering (LLS) revealed that the prepared nanoparticles had a spherical shape, and the size of PCLLA nanoparticles (similar to 85 nm) was smaller than that of the PCL and PLLA nanoparticles. The experiment of in vitro drug release showed that the drug release rate from PCLLA nanoparticles was slower than that from PCL and PLLA nanoparticles, and the release profile of PCL6/LA4 nanoparticles appeared to follow zero order kinetics. These results suggested that the polymer composition made a great influence on the nanoparticle size and drug release behavior. (C) 2000 John Wiley & Sons, Inc.