The beneficial effects of Omega-3 poly-unsaturated fatty acids (n-3) as nutritional components is related to its anti-inflammatory effects. In spite of anti-inflammatory effects, controversial results of omega-3 fatty acids have been observed in the treatment of ulcerative colitis (UC). This systematic review was undertaken to assess the therapeutic effects of omega-3 fatty acids supplementation on UC. A search in PubMed database with specified MeSH terms was conducted and the RCT was selected that were published up to November 12th, 2017 and in English language evaluating the effect of oral supplementation of n-3 fatty acids in adult Patients suffering from active and inactive UC. The trials assessing the improvement of UC and reporting the index as a result (8 trials), but trials with consumption of dietary supplements enriched with n-3 fatty acids were excluded. These trials evaluated the improvement of UC through intervention by using n-3 fatty acids with different outcomes such as clinical response, disease activity index (DAI), endoscopic and histology scores, reduction in corticosteroids consumption in patients achieving remission after treatment and time in remission. The evaluation data indicated reduction in corticosteroids dosage in UC patients during fish oil treatment in all three related trials that assessed this index. Only one of these trials reported statistical significant changes in dosage requirement of corticosteroids. Based on controversial results that were obtained from this systematic review, it was not expressed conclusively that taking omega -3 PUFA supplements in the treatment of UC is helpful and need more studies in this field.