Objective: To optimize ultrasonic-assisted extraction process of total flavonoids from Clinacanthus nutans and evaluate its in vitro antioxidant activity. Method: Based on single-factor tests

Box-Behnken design model was applied to investigate effects of ethanol concentration

temperature and ratio of solid to liquid on extraction efficiency. Besides

in vitro antioxidant activities were evaluated simultaneously. Result: Optimum extraction process was as following:extracted 30 min with 42 times the amount of 67% ethanol at 53 ℃;yield of total flavonoids was as high as 12.15 mg·g-1 under these conditions

whose deviation was 1.86% by comparing with the predictive value of 12.38 mg·g-1.Total flavonoids from C. nutans turn out to be equipped with strong reducing power with an C50(the concentration when absorbance was 0.5) of 179.03 mg·L-1

as well as significant scavenging activity to DPPH free radical and ABTS free radical with half inhibitive concentration(IC50) of 89.72

51.92 mg·L-1

respectively.Extract also demonstrated significant protective effect against H2O2-induced hemolysis with an IC50 of 237.22 mg·L-1. Conclusion: Total flavonoids from C. nutans have desired antioxidant activity.