Abstract： Objective To compare the clinical disease activity index (CDAI) and simplified disease activity index (SDAI) as well as disease activity score 28 (DAS28) by assessing the activity of rheumatoid arthritis (RA) and to explore which one is better.Methods Two hundred RA patients were enrolled.Swelling joint counts (SJC),tenderness joint counts(TJC ),patient's and doctor's global assessment of disease activity based on visual analogue scale (PGA,PhGA),health assessment questionnaire (HAQ) were recorded and the erythrocyte sedimentation rate (ESR),C reaction protein (CRP) of each patient were tested.DAS28,CDAI,SDAI of all patients were calculated for all patients.Statistical analysis was carried out by Pearson correlation for the association between DAS28 and the above parameters,as well as CDAI,and SDAI.We created 4 patient groups based on DAS28,CDAI and SDAI ranks and used kappa statistics to assess the level of overall agreement of different disease activity categories between any of the two indices above for individual patients.We assessed the discriminating validity using receiver operating characteristic (ROC) curve analysis to compare the ability of the CDAI and SDAI to discriminate betwecn patients with remission, low and moderate,high disease activity.Results Of all the patients,CDAI(17.2±11.1) and SDA[(19.1±11.6) were correlated with DAS28 (4.3±1.5),the correlation coefficients were 0.876,0.861 (P＜0.05) respectively.CDAI and SDAI were correlated with HAQ (0.6±0.7),as well as DAS28.The correlation coefficients were 0.522,0.523,and 0.482 (P＜0.05).The Kappa of CDAI and SDAI was 0.777.The Kappa of CDAI and DAS28,SDAI and DAS28 were 0.482,0.394.The areas under ROC of CDAI and SDAI were 0.906,0.888 if DAS28 was used as the gold standard.Conclusion CDAI and SDAI as well as DAS28 can be used to assess the activity of RA and both are better correlated with HAQ than DAS28.Though there is no CRP in CDAI when compared with SDAI,CDAI has very good overall agreement with SDAI and the overall agreement of CDAI and DAS28 is better than SDAI and DAS28.In addition, CDAI is better in discriminating between patients with remission/low and moderate/high disease activity.So,CDAI is a simple,convenient,accurate,quick assessment tool and is suitable for daily application.