Abstract: Poly lactic acid(PLA) microparticles were prepared by a supercritical fluid method, called the aerosol solvent extraction system method.The effects of pressure, temperature, PLA solution concentration and its flow rates,and PLA molecular weight (Mw) on the morphology and particle size (PS) of the resulting PLA microparticles were investigated.The results showed that a viscous film was found at sinter filter fixed on the bottom of the precipitator when Mw was 100000, or PLA solution concentration was 1 g•ml^-1, when Mw was less than 100000 or PLA solution concentration was smaller than 1 g•ml^-1, PLA microparticles were produced.In the scope of study here, pressure and temperature were the most significant factors on PS, and increasing pressure, or decreasing temperature could lead to a significant decrease in PS.At 20 MPa，35℃，PLA flow rate 240 ml•h^-1，concention 0.1 g•ml^-1,M_w=10000, the PLA microparticles with a PS of 200～600 nm were successfully obtained.XRD suggested that there was no significant change in crystalline type, but significant decrease in the crystallinity of the resulting PLA products was observed.