Abstract: Objective To explore the correlation between severity of ulcerative colitis（UC） and expression of IL-21 and its receptor（IL-21R）. Methods Peripheral blood samples were obtained from 34 patients with UC including 10 cases in low severity group,8 cases in medium severity group,6 cases in high severity group,and 10 cases in remission group. Thirty healthy subjects enrolled served as a control group. The level of serum IL-21 was measured by enzyme-linked immunosorbent assay（ELISA）. Expression of IL-21R in peripheral blood CD4⁺,CD8⁺ T,B and NK cells was analyzed by flow cytometry. Results The levels of serum IL-21 and expression of IL-21R in peripheral blood CD4⁺,CD8⁺ T,B and NK cells were significantly higher in UC patients than in the healthy controls（P＜0.01）. Conclusion IL-21 and its receptor play an important role in the pathogenesis of UC,and may be associated with intestinal mucosal injury.