Vitamin D3 has a role in many autoimmune diseases and appears to play a function in controlling Rheumatoid Arthritis (RA). The aim of this study is to evaluate the relationship between serum level of vitamin D and RA disease activity score. The serum level of vitamin D in 75 RA patients referred to the rheumatology clinic of Rasoul-Akram hospital was measured. Patients were classified into low, moderate and high RA activity groups based on the DAS-28 criteria (Disease Activity Score in 28 joints) and the mean values of serum vitamin D were compared between the three groups.The mean serum levels of vitamin D in high activity group (17.057±7.7 mg/ml) was significantly less than moderate (30.5±11.3 mg/ml) and low (36.7±19.5 mg/ml) activity groups (P<0.001). The outcome of this study shows that serum level of vitamin D is inversely correlated with the activity of RA.