In this study, the quantitative analysis of saikosaponins from Radix Bupleuri in China was performed by high performance liquid chromatography. Saikosaponin-a and -d were converted completely into saikosaponin-b1 and -b2 by mild acid treatment. Distinctive measuring of these converted diene-saponins provided a rapid and selective method for the determination of saikosaponin-a and -d in commercial samples of Radix Bupleuri. The conditions of extraction and conversion of saikosaponins were optimized using orthogonal design L9(3(4)). The HPLC analysis was performed on ODS-C18 column with a flow rate of 1.0 ml/min and detection wavelength of 250 nm. Well resolved chromatograms of saikosaponin-b1 and -b2 were obtained with an isocratic elution of acetonitrile : 1% formic acid water (37.5 : 62.5). Calibration curves of saikosaponin-b1 and -b2 were linear in the range of 4.9-98.0 microg/ml and 3.5-71.0 microg/ml, respectively. The average recovery of saikosaponin-b1 and -b2 were 98.3% (RSD = 3.1%) and 96.4% (RSD = 1.8%), respectively. Seventeen samples of different species and habitats of Radix Bupleuri were analyzed by the developed HPLC method.