Objective: We aimed to investigate the serum level of interleukin-35 (IL-35) in patients with rheumatoid arthritis (RA).The relationship between IL-35 and interleukin-17 (IL-17), immunological index markers and disease activity score were also studied, The possible roles IL-35 may play in the pathogenesis of RA were also discussed.Methods: The serum samples of newly diagnosed RA patients without any drug therapy and relapsed RA patients without glucocorticoid treatment were collected in the study.Thirty healthy volunteers were recruited as the control, serum samples were collected.ESR, CRP, anti-CCP, RF, tender joint count,swollen joint count, Ritchie articular index and DAS28 score of the patients were measured and recorded.IL-35 and IL-17 had been detected in all 74 cases via ELISA.Result: The serum IL-35 level in RA group was lower than that in health group, and this difference reached statistical significance (P<0.05);serum IL-17 level in RA group was higher than that in health group,and this difference reached statistical significance (P<0.05);the serum levels of IL-35 in RA patient negatively correlated with IL-17 (r=-0.324, P=0.035),tender joint count (r=-0.509, P=0.001) ,swollen joint count (r=-0.373, P=0.001) and Ritchie articular index, it had reached statistical significance (P=0.14).Serum IL-35 level had not correlated with DAS28 scores (r=-0.25, P=0.102) , RF (r=-0.015,P=0.925) , ESR (r=0.022,P=0.887) , CRP (r=0.135,P=0.383) , anti-CCP (r=0.143,P=0.353).Conclusion: The expression of serum IL-35 levels in RA patients are reduced and it negatively correlated with serum IL-17 levels ,tender joint count, swollen joint count and Ritchie articular index.It means that IL-35 may play a key role in the pathogenesis and process of RA.