BACKGROUNDBupleuri Radix (in Chinese Chaihu), the dried roots of Bupleurum Chinense DC, is a traditional Chinese medicine widely used to treat fever, hepatitis, jaundice, nephritis, dizziness. When baked with vinegar, its effect is more focused on liver related disease. This paper was undertaken to determine the best vinegar amount in the processing and explore its key efficacy components.METHODSHepatoprotective effects of Radix Bupleuri after processing with different amount of vinegar (1:5, 2:5, 3:5) were investigated on liver hurt rats, and the change of constituents were analyzed by ultra-performance liquid chromatography tandem mass spectrometry (UPLC-MS/MS).RESULTSWith the increasing amount of vinegar, the hepatoprotective effects of vinegar-baked Radix Bupleuri (VBRB) and the content of saikosaponin b2 increased.CONCLUSIONThese results suggested that vinegar amount in the process affected the pharmacological effect of VBRB significantly and saikosaponin b2 may be the key efficacy component of it.