Abstract: A micellar electrokinetic capillary electrophoresis (MEKC) method was developed for the determination of three pharmaceutically important coumarins: osthole (OS), imperatorin (IM) and isoimperatorin (IS) in Cnidium monnieri (L.) Cusson and its medicinal preparation (capsule of ‘Sanwushenwei’). A buffer solution (pH 10.50) composed of 10 mmol/L sodium borate, 20 mmol/L sodium dodecyl sulfate (SDS) and 10% methanol was found to be the most suitable electrolyte for this separation. The temperature and the applied voltage were 25 ℃ and 25 kV, respectively. Regression equations revealed linear relationships (correlation coefficients: 0.999 8, 0.999 4 and 0.998 9) between the peak area of each compound (OS, IM and IS) and its concentration. The relative standard deviations of migration times and peak areas were <1.86 and 3.82% within one day, respectively. The effects of pH value, surfactant (SDS) concentration and organic modifier on the migration behavior were also studied. By this way, the contents of active coumarins in the extracts of the Cnidium monnieri . Cusson and its medicinal preparation were successfully determined within 8 min.