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 CO2. In this work, we studied the effect of CO2 density, solution injection rate, and CO2 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 CO2 densities, there was not a consistent tendency. From variation of solution injection rate and feed rate of CO2 we found out that the relative velocity difference between CO2 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 CO2 density. Particles in which DCM was used as the solvent for PLLA were finer than the others.