In order to facilitate the quality control of some selected Korean thistles (Cirsii Herb), Cirsium japonicum var ussuriense, C. japonium var spinosissimum, C. setidens, C. pendulum, C. nipponicum, Carduus crispus, and Breea segetum, a simple, accurate and reliable high performance liguid chromatography method was developed for the simultaneous determination of the six flavonoids: luteolin 5-O-glucoside (1), luteolin 7-O-glucoside (2), hispidulin 7-O-neohesperidoside (3), luteolin (4), pectolinarin (5), and apigenin (6), which were selected as the chemical markers of the thistles. Separation was achieved on an Agilent Eclipse XDB-C18 column with a gradient solvent system of 0.1% trifluoroacetic acid aqueous-methanol at a flow-rate of 1.0 mL/min and detected at 254 nm. All six calibration curves showed good linearity ($R^2$ > 0.991). The method was reproducible with intra- and inter-day variations of less than 6%. The recoveries were in the range of 90.01-100.05%. This analysis method was successfully utilized to quantify the six flavonoids in the 22 batches of the thistles. The results demonstrated that this method is simple, reliable and suitable for the quality control of this medicinal herb.