Abstract: This study aimed to investigate the effect of Huangqin Tang (HQT) on oxidative stress associated with ulcerative colitis in rats, and to explore its antioxidant mechanism. After approved by Institute of Chinese Materia Medica Ethics Committees in China Academy of Chinese Medical Sciences, the rats were given 2,4,6-trinitrobenzenesulfonic (TNBS)/ethanol mixture to induce the ulcerative colitis (UC), and were randomly divided into normal group, model group, the salazosulfapyridine (SASP) group, and high, middle or low dose (20, 10, 5 g·kg^-1) of HQT groups. After 5 days of treatment, the activity of catalase (CAT) from micrococcus lysodeikticus, glutathione peroxidase (GSH-px), myeloperoxidase (MPO), superoxide dismutase (SOD) were detected by biochemical assays. The levels of lipid peroxide (LPO) were detected by ELISA. The positive protein expression of nuclear factor erythroid-2-related factor 2 (Nrf2) were detected by immunohistochemistry method and the downstream antioxidant enzymes of Nrf2 were determined by Western blot analyses. The levels of SOD, CAT and GSH-px activities in the normal group were significantly higher than the model group, while the serum MPO activity in the model group was obviously increased (P<0.05 or P<0.01). Compared with the model group, there was a significant difference in the activity of CAT in the high and middle dose groups of HQT (P<0.05 or P<0.01), the activity of GSH-px in the high, middle and low dose groups of HQT were apparently higher than the model group (P<0.05); The serum levels of LPO in the model group were significantly higher than those in the normal group (P<0.05), while the up-regulating effects on LPO were reversed by the high and middle dose groups of HQT (P<0.05). The expression of Nrf2 in the high-dose group of HQT and SASP group was statistically significant (P<0.05). The results of Western blot showed that compared with the model group, each of the HQT and SASP group could increase the heme oxygenase (HO-1) and NADPH:quinone oxidoreductase 1 (NQO-1) expression in a dose-dependence manner. HQT has significant anti-oxidative stress and obviously improves the signs, mental status and defecation of UC rats. The mechanism of action for HQT maybe related to activate the Nrf2 pathway and increase the expression of Ⅱ phase metabolic enzymes such as HO-1 and NQO-1, reduce the content of LPO and MPO in serum and enhance the activity of SOD, CAT and GSH-px.