Abstract: Objective To measure the cartilage oligomeric matrix protein（COMP） level in serum and synovial fluid（SF） and to study its significance in diagnosis of rheumatoid arthritis（RA）. Methods COMP levels in serum and SF,as well as serum rheumatoid factor（RF）,anti-cyclic citrullinated peptide antibodies（CCP）,C-reactive protein（CRP） level,and erythrocyte sedimentation rate（ESR） were measured by ELISA in RA and control groups.Arthralgia and swelling index and X-Ray classification were detected.Serum COMP level in RA group was measured after treatment. Results The serum COMP level was significantly higher in RA group than in control group（P＜0.01）.A significant difference was found in serum COMP level between patients with active and inactive RA（P＜0.01）.The COMP level was higher in synovial fluid than in serum（P＜0.05）.There was a positive correlation between COMP level in serum and SF（r=0.73,P＜0.01）.The COMP level was positively correlated with ESR,CRP levels and arthralgia and swelling index（P＜0.01）.A significant difference was observed in serum COMP level before and after treatment（P＜0.01）. Conclusion COMP level is related with the activity of RA and arthrosynovitis,and can thus be used as a serological index of the disease progress and improvement.