Abstract: Objective To assess the clinical value of monocytes and monocytes-to-lymphocytes ratio(MLR) in clinical and endoscopic severity of ulcerative colitis(UC). Methods Total 102 patients with UC and 100 healthy controls were enrolled in the First Affiliated Hospital of University of Science and Technology of China from January 2018 to September 2019. The expression of monocytes and MLR in UC patients with different clinical and endoscopic severity was retrospectively analyzed. Results MLR, white blood cells, neutrophils and platelets in UC patients were significantly higher than those in the healthy controls(all P<0.05). Monocytes was positively correlated with MLR, WBC, ESR, CRP and platelets(all P<0.05), while MLR was positively correlated with WBC and CRP(all P<0.05). According to modified Truelove and Witts' s severity criteria and modified Mayo score, monocytes and MLR obviously increased in moderate and severe patients(all P<0.05). According to Mayo endoscopy subscore and Montreal classification, MLR increased in the Score 3 group compared with the 1 or 2 groups, and significantly increased in the E3 group compared with the E1 or E2 groups(all P<0.05). The ROC curve analysis showed that the area under the curve of monocytes count and MLR were 0.740(95% CI: 0.672-0.807) and 0.750(95% CI: 0.683-0.817), respectively, with statistical significance(all P<0.001). The optimal critical values of monocytes and MLR were 0.385×10⁹/L(sensitivity: 81.37%, specificity: 53.00%) and 0.235(sensitivity: 75.49, specificity: 66.00%), respectively. Conclusion Monocytes and MLR are increased in UC patients and significantly correlated with the clinical and endoscopic severity of UC.