Taxol-loaded nanoparticles were prepared by solvent-evaporation technique with methoxy-added methoxy poly(ethylene glycol)-b-poly(epsilon-caprolactone) (MPEG-b-PCL) (PEG wt % = 18.5%) as drug carrier and MPEG-b-PCL (PEG wt % = 86.8%) as the additive in the outer aqueous phase. The MPEG-b-PCL copolymers were synthesized via ring-opening polymerization of epsilon-caprolactone using Novozym 435 as biocatalyst. As a newly used additive in the outer aqueous phase, only 0.1% (w/v) MPEG-b-PCL with relatively high-PEG content was needed. The effects of feed ratio of taxol to copolymer (w/w, 5-20%) on the characters of nanoparticles and in vitro drug release were investigated. The fabricated nanoparticles reached the highest encapsulation efficiency of 52.5% +/- 3.9% when the feed ratio was 10%. The nano-particles presented a core-shell structure in aqueous solution, and their hydrodynamic diameter ranged from 75 to 105 nm. Transmission electron microscopy and atomic force microscope investigations exhibited that the nanoparticles had fine spherical shape and narrow particle size distribution. The size of nanoparticles did not change distinctly after incubation in pH 7.4 phosphate-buffered solution (PBS) and pH 7.4 PBS with 10% fetal bovine serum at 37 degrees C for 24 h. The nanoparticles with drug loading around 5% could achieve a sustained drug release for 7 days. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 121: 2386-2393, 2011