The main target of this study is to assess the safety and efficacy of β-D-Mannuronic acid in patients suffering from Rheumatoid Arthritis. Β-D-Mannuronic acid which is an anti-inflammatory agent, is ranked in the family of nonsteroidal anti-inflammatory drugs. This agent has expressed qualified therapeutic effects with the greatest tolerability and safety in different experimental models like experimental model of Multiple schlerosis, Rheumatoid arthritis, nephrotic syndrome and acute glomerulonephritis. In this randomized controlled trial, 203 patients afflicted by Rheumatoid Arthritis, diagnosed based on the American College of Rheumatology (ACR) Diagnostic Criteria that have active disease, will be examined. Furthermore, these patients should not have other concomitant diseases such as Hepatic, renal, cardiovascular diseases or malignancies. Written informed consent will be signed by the patients. Then Patients will be randomly divided into two groups (Treatment and Control group). Treatment group (112 patients) will receive β-D-Mannuronic acid 1500 mg/day (three 500 mg tablets/day)with conventional immunosuppressive drugs and Control group (91 patients) ) will receive placebo with conventional immunosuppressive drugs orally for 12 weeks. Medical history and clinical parameters including the serum level of CRP, ESR, RF and Anti CCP will be evaluated at baseline and 12 weeks after treatment. The manner of blinding in this study is so neither participants in the study nor the persons who perform the test will be aware of the intervention. In order to allocate the patients randomly into two groups mentioned above, at first 29 blocks of 7 with C and T letters (The letters indicate the Treatment and Control groups) are created. 4 patients of each block belong to the treatment group and 3 patients belong to the control group. Then the blocks are randomly selected and arranged to reach a sequential combination of 203 letters. Each letter will be placed in a sealed packet according to the obtained sequence