Purpose: In inflammatory bowel disease increased asymmetric dimethylarginine (ADMA) levels could inhibit nitric oxide (NO) synthase. Vitamin D may increase activity and expression of endothelial NO synthase, which could be done through its possible mechanism of decreasing ADMA levels. The aim of this study is to investigate the possible effect of Vitamin D3 on serum ADMA levels in ulcerative colitis (UC) patients. Methods: Ninety mild to moderate UC patients were randomized. Each patient received one single muscular injection of 300,000 IU (7500 μg) Vitamin D3 (Vitamin D group) or 1 ml normal saline (Placebo group). At baseline and 90 days after the intervention measurements were done. Data were analyzed using independent t-test and analysis of covariance. Baseline correlations were assessed by Pearson and Spearman correlation coefficients. Results: Following data analysis of 86 participants (40 in placebo and 46 in vitamin D group), there was no correlation between baseline ADMA with baseline vitamin D, ESR and hs-CRP at baseline (p = 0.77) and at the end of study (p = 0.82). Serum ADMA levels were not statistically different between two groups. Adjustment for baseline ADMA levels and baseline body mass index (BMI) did not change the results. With subgroup analyses based on gender and vitamin D level no statistical differences in ADMA levels between two groups were found. Conclusions: In this study, we found no significant changes in serum ADMA levels 3 months following a high dose vitamin D administration in mild to moderate UC patients. Further studies in vitamin D deficient patients are needed.