Saikosaponins a, c, d, b1 and b2 which are the trace constituents in the crude extract of Bupleuri Radix can be separated successfully by the on-line sample stacking technique (Sweeping method) of capillary electrophoresis. The method uses aqueous phosphoric acid with a pH lower than 2 to prepare the extracting solvent and buffer solution of the same electric conductance. The former is prepared with methanol and the latter is prepared by adding SDS, methanol and acetonitrile. Bupleuri Radix extract analyzed by CE-sweeping method gives better result than by MEKC method with a higher separation efficiency, lower detection limit and ability to detect five saikosaponins. Applied to the analysis of two Chinese herbal formulas, CE sweeping method is found to be able to determine the contents of saikosaponins a and b1 successfully in Minor Bupleurum Combination and Major Bupleurum Combination, both of which contain Bupleuri Radix as the imperial drug. However, the migration times and resolution of the two marker substances, saikosaponins a and b1, were changed obviously by the complicate constituents in Chinese herbal preparations.