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Background: Rheumatoid arthritis (RA) is one of the most prevalent autoimmune diseases in the world. Some of the researches have suggested that the serum vitamin D level may relate to disease activity. The current study was designed to identify the correlation between serum vitamin D level and the disease activity index (DAI). Materials and Methods: In this sectional study, 108 patients (diagnosed based on the American Committee of Rheumatology criteria) were enrolled. In all cases after determining the serum vitamin D level and ESR, complete joint examination were done. The normal range of vitamin D was 47.7 - 144 nm/lit. Then, the specified disease activity form (DAS- 28) and visual analog scales (VAS) were filled out. The correlation between disease activity index, (based on VAS, tendered or swelled joints and ESR) and serum vitamin D was analyzed using Chi square, Mann Whitney and t-tests. Results: Seventy nine (73.14%) out of 108 patients had normal serum vitamin D and 29 (26.86%) had low serum vitamin D level. The mean age of patients with normal and low serum vitamin D level were 52.22±11.6 and 48.48±12.51, respectively (P=0.075). The mean DAI in normal and low vitamin D patients were 3.75±1.37 and 5.19±1.56, respectively (P=0.00). The mean number of swollen joints in normal and low vitamin D patients was 1.24±1.39 and 3.65±3.3, respectively (P=0.001). The mean number of tender joints in normal and low vitamin D patients were 6.84±4.41 and 9.44±3.62, respectively (P=0.042). The mean ESR in normal and low vitamin D patients were 19.40±12.40 and 37.91±21.72, respectively (P=0.002). Moreover, the mean VAS in normal and low vitamin D patients was 23.29±19.42 and 50.68±30.78, respectively (P=0.003). Conclusion: The more active the RA, the less serum vitamin D level. [ABSTRACT FROM AUTHOR]