Abstract: OBJECTIVE To explore the treatment mechanism of Gancao Xiexin Decoction on patients with ulcerative colitis(UC) by observing the serum levels of interleukin-17(IL17)， interleukin-23(IL23) before and after the treatment. METHODS 60 patients with UC were randomly divided into control group and treatment group， with 30 patients in each group. Patients in the control group were given oral sulfasalazine， while those in the treatment group were given Gancao Xiexin Decoction combined with sulfasalazine. After 8 weeks treatment， the clinical efficacy evaluation was done. To be specific， the disease activity score was calculated according to Mayo Score， and the quality of life was measured by IBD Questionnaire(IBDQ) before and after the treatment. Besides， peripheral venous blood samples of patients in both groups were collected and the serum levels of IL17， IL23 were measured with enzyme-linked immunosorbent assay(ELISA). RESULTS The total effective rate of the treatment group was 93.3%， which was evidently superior to that of 80% in the control group(P<0.05); The Mayo score of both groups went down after the treatment(P<0.01)， and the score of the treatment group was much lower than that in the control group(P<0.01). The symptom scores such as intestinal tract symptom， constitutional symptom， emotional capability， social competence as well as the total scores of the IBDQ in the treatment group were higher than those in the control group(P<0.05~0.01). The serum levels of IL17、IL23 in both group declined after the treatment， while the decrease in the treatment group was more obvious compared with the control group(P<0.01). CONCLUSION Gancao Xiexin Decoction combined with sulfasalazine shows significant efficacy in improving the clinical symptoms and life quality of patients with UC. One of the action mechanisms of Gancao Xiexin Decoction in treating UC might be its effective role in decreasing the serum levels of IL17， IL23 and participating in immune regulation.