Abstract: Fine griseofulvin particles formation by rapid expansion of supercritical solutions (RESS) is studied. An RESS experiment apparatus is set up based on NOVA supercritical experiment equipment. The throttle orifice was made by a stainless steel foil, which was drilled by laser. The diameter of the orifice was between 10—50 μm and its length is 400 μm. The nozzle was heated by wire and the temperature could be controlled and indicated by a thermocouple. The expansion chamber was made of glass equipped with a heating jacket, therefore the expansion backpressure can only be maintained in atmospheric pressure. When the solution flow through the nozzle to throttle and spray into the expansion chamber, griseofulvin was then deposited in the chamber. The effect of solute concentration was researched by change in temperature of the dissolution column and therefore the second solvent pipeline was not needed. All samples were observed by a microscope and a camera was used for recording. Several operation parameters were studied. At first, sampling positions in expansion chamber were tested and the results showed that it was not sensitive for the diameter of griseofulvin. Then, the effect of the diameter of the orifice, the pre-expansion temperature, the dissolution pressure, the temperature of expansion chamber, the nozzle heating and the concentration of griseofulvin on the diameter of griseofulvin were studied. The results show that the smaller the orifice diameter is, the smaller the griseofulvin particle is.The higher the temperatures are, the smaller the particles are.The solubility of griseofulvin in supercritical CO 2 is in the converse condensation region. Thus, higher temperature will cause high supersaturation rate and increase the nucleation rate.It means that the particle size will be reduced. Moreover, it must be some degree of supersaturating in pre- expansion section because the fluid should be heated to a higher temperature before entering the nozzle to keep the fluid in single-phase area at the throttle. The supersaturating would have caused a few griseofulvin deposited before the nozzle but the experiment confirmed that at 25 MPa griseofulvin will never be deposited even though some supersaturating values exist. Also,as indicated by other papers,the experiment results pointed out that reducing the solute concentration before nozzle will form large particles. Therefore, 0.8? ?.8 μm griseofulvin particles can be formed by RESS,when the dissolution temperature ranges from 308.2 to 313.2 K,the dissolution and pre- expansion pressure is 25 to 30 MPa,the pre-expansion temperature and nozzle temperature is 308.2 to 323.2 K,and the expansion chamber temperature ranges from 303.2 to 313.2 K .