Methoxy poly(ethylene glycol)-b-poly(L-lactide) (MPEG-PLLA) diblock copolymer was synthesized via ring-opening polymerization, and MPEG-PLLA microparticles were then prepared using an aerosol solvent extraction system (ASES) technique with compressed carbon dioxide as antisolvent. The MPEG-PLLA microparticles were prepared at temperatures ranging from 25 ℃ to 55 ℃ and at pressures from 85 bar to 150 bar. The concentration of MPEG-PLLA copolymer, solution flow rate, and CO2 flow rate were adjusted to be 0.5-3.0% (w/v), 0.3-1.0 mL/min, and 19 g/min, respectively. Relatively small spherical microparticles were prepared in the subcritical region at 25 ℃ , while agglomerated particles were obtained at temperatures above the critical point. The mean particle sizes of the MPEGPLLA microparticles prepared by the ASES varied from 9.53 μm to 46.9 μm depending upon the operating conditions.