Abstract: A liquid chromatography tandem mass spectrometry (LC-MS/MS) method was developed for the simultaneous determination of the type I saikosaponin a,c,d and the type II saikosaponin b₂ in crude radix bupleuri and radix bupleuri processed by vinegar.The analytes were well separated on a Kinetex^TM C₁₈ column (50 mm×4.6 mm，2.6 μm) using gradient elution.The analytes and the internal standard were analyzed by monitoring the fragmentation transition pair of anionic adducts to deprotonated molecules in negative electrospray mode.The validated method was proved to be sensitive,precise,accurate and selective,and could be applied for the quality control of crude and processed radix bupleuri by vinegar.The results of quantification indicates possible chemical transformation of the type I saikosaponin to their structural isomers of the type II,and the quality control of radix bupleuri processed by vinegar should take consideration of both type I and II saikosaponins.