Background: Ulcerative colitis (UC) is one of the chronic diseases which is increasing in prevalence and patients suffer from illness are-ups. UC standard regimen treatment has various side effects besides the e cacy, so there is an interest in administering complementary medicine to reduce adverse effects and increase the e cacy, as well. The aim of this study was to evaluate the e cacy and anti-in ammatory effect of Thymus kotschyanus as an additive treatment in a randomized double-blind placebo-controlled trial of UC patients.Methods: Thirty UC out-patients with mesalazine regimen treatment that ful lled the inclusion criteria were participated in a 12-week trial and were randomly chosen for the treatment and control group.Fifteen patients were administered a placebo as a control and 15 patients were administered received Thymus kotschyanus extract by a dose of 0.5 grams in a day in the treatment group. Laboratory tests were performed at baseline and week 12. The primary outcome was reduction in fecal calprotectin as the main intestine in ammatory marker. Likewise, reduction in SCCAI, SIDBQ, and SEO indices were considered as the secondary aims. The primary outcome was a reduction in SCCAI as the main intestine in ammatory marker. Likewise, reduction in fecal calprotectin, SIDBQ, and SEO indices were the secondary aims.Results: Fecal calprotectin Comparing the treatment and placebo groups at week 12 indicated a was decreased in fecal calprotectin by 54.74% in the treatment group, as compared with placebo group at week 12 (p=0.02). and alsoA signi cant difference reduction was shown in SCCAI was also shown between two study groups (p=0.01). Thymus kotschyanus extract was safe and no severe side effects were reported.Conclusion: Administration of Thymus kotschyanus revealed improvement in UC symptoms by the intestinal anti-in ammation effect of the plant and could be suggested as a potential additive treatment in UC patients. The study protocol has been registered under the identi cation code: IRCT20200406046965N2.