Tetracycline hydrochloride (TTC) was micronized by an Aerosol Solvent Extraction System (ASES) using supercritical CO2. The effects of solvent, pressure and temperature of CO2, solution concentration, and solution feed rate on particle size were investigated. Mean particle sizes of processed TTC were 0.16-0.31 μm, but the morphologies of processed particles were affected by agglomeration between the primary particles. Mean particle sizes of unprocessed TTC were ca. 200 μm and the shapes were irregular with rough surfaces. Especially, particle sizes increased from 0.18 to 0.31 μm as CO2 temperature increased. In addition, particle sizes increased from 0.18 to 0.23 μm as TTC concentration increased. Powder X-Ray diffractometry revealed that processed particles were amorphous whereas unprocessed particles showed strong crystallinity.