A rapid expansion of supercritical solution (RESS) technology was presented for the micronization of Chinese medicinal material. Magnolia bark extract (MBE) obtained by supercritical carbon dioxide (scCO₂) extraction technology was chosen as the experimental material. RESS process produced 4.7 μm size MBE microparticles (size distribution, 0.2—24.1 μm), which was significantly smaller than the 55.3 μm size particles (size distribution, 8.3—102.4 μm) obtained from conventional mechanical milling. Dissolution rate study showed that drug dissolution was significantly enhanced by the RESS progress. At 90 min, the amount dissolved of mechanical milling MBE was 6.37 mg·l⁻¹, which was significantly lower than that of micronized MBE (14.77 mg·l⁻¹), according to the results of ANOVA ( p <0.01). The effect of extraction temperature (30, 40, 50 °C), extraction pressure (200, 250, 300 bar) and nozzle size (50, 100, 200 μm) on the size distribution of microparticles was investigated. The characteristics of microparticles were also studied by differential scanning calorimetry (DSC), infrared spectroscopy (IR), scanning electron microscopy (SEM), and image analysis. This study demonstrates that RESS is applicable for preparing microparticles of MBE at low operating temperature; the process is simple without residual solvent.