The effects of ultrasonic-assisted extraction factors for the main phenolic compound (quercetin) from Hypericum perforatum L. were optimized using the Box-Behnken design (BBD) combined with response surface methodology. The BBD was employed to evaluate the effects of extraction temperature (30-70 degrees C), extraction time (20-80min), methanol concentration (20-80%, v/v), and HCl concentration (0.8-2.0M) on the content of one of the major phenolic compounds of quercetin. The extracts were analyzed by high performance liquid chromatography (HPLC). The major phenolic compounds of H. perforatum were isolated and the antioxidant capacity and total phenol content were determined in crude extract and fractions. The optimum conditions were determined as extraction temperature 67 degrees C, extraction time 67min, methanol concentration 77% (v/v), and HCl concentration 1.2M. The predicted content of quercetin was 10.81mg/g dried plant under the optimal conditions and the subsequent verification experiment with 11.09mg/g dried plant confirmed the validity of the predicted model. The isolated compounds were identified as quercetin, cyanidin, protocatechuic acid, and kaempferol.