A rapid expansion from supercritical solution into aqueous solution (RESSAS) technology was presented for the micronization of Chinese medicinal material. Magnolia bark extract (MBE) obtained by supercritical carbon dioxide (scCO(2)) extraction technology was chosen as the experimental material. RESSAS process produced 303.0 nm nanoparticles (size distribution, 243.6--320.5 nm), which was significantly smaller than the 55.3 mu A mu m particles (size distribution, 8.3--102.4 mu A mu m) prepared by conventional mechanical milling. The effect of process parameters, including extraction temperature (30 degrees A degrees C, 40 degrees A degrees C, 50 degrees A degrees C), extraction pressure (200, 250, 300 bar) and nozzle size (50, 100, 200 mu A mu m), on the size distribution of nanoparticles was investigated. The characteristics of nanoparticles and materials were also studied by scanning electron microscopy (SEM) and laser light scattering (LLS). This study demonstrates that RESSAS is applicable for preparing nanoparticles of MBE at low operating temperature; the process is simple without any residual solvent.