The ASES (aerosol solvent extraction system) process, which is one of the SAS (supercritical antisolvent) processes, was selected to recrystallize PLLA into submicrometer particles. In the ASES process, there are two key factors. One is atomization for fine droplets, and the other is mass transfer of droplets during precipitation in the vessel, which causes nucleation and growth of particles. They are affected by several elements such as temperature, pressure, density, concentration, injection rate of solution, and feed rate of CO 2 . In this work, we studied the effect of CO 2 density, solution injection rate, and CO 2 feed rate on atomization and mass transfer. In addition, to investigate the influence of the solvent on particle production, DCM (dichloromethane), THF (tetrahydrofuran), and 1,4-dioxane were used as solvents for PLLA. In the variation of CO 2 densities, there was not a consistent tendency. From variation of solution injection rate and feed rate of CO 2 we found out that the relative velocity difference between CO 2 and the PLLA/DCM solution was an important factor for fine PLLA particles. The particles increased in size with relatively high temperature and pressure for fixed CO 2 density. Particles in which DCM was used as the solvent for PLLA were finer than the others.