Abstract: OBJECTIVE To establish an Ultra Performance Liquid Chromatography (UPLC-MS) method to analyze the flavonoids constituents in Herba Hyperici Japonici.METHODS The analysis was performed on an Alltima C₁₈ chromatographic column with a gradient solvent system of acetonitrile-0.8% acetic acid at a flow rate of 0.8 mL/min. The detection wavelength was 254 nm and the column temperature was 30 ℃. The data was collected under the negative electrospray ion mode.RESULTS 8 flavonoids constituents being separated and identified， six of them were detected simultaneously. The result shows the detected flavonoids constituents have good linearity between concentration and peak area (r²>0.998 2)， and the average recoveries of the method were between 97.6%~99.7%.CONCLUSION Being simple， accurate and reproducible， UPLC-MS can be used for evaluating the quality of Herba Hyperici Japonici.