Objectives: DM is a well-known risk factor for tuberculosis(TB) and patients with DM and TB have increased morbidity, higher probability of treatment failure, or recurrence. According to the recent study, Metformin, which is one of the most important anti-diabetic drugs, can inhibit intracellular growth of mycobacteria by inducing mROS. Therefore, this study was performed to determine the effect of Metformin on the treatment result of tuberculosis, especially in patients with DM. Methods: This was a retrospective cohort study. The study population consisted of culture-positive pulmonary tuberculosis patients newly diagnosed between 2011 and 2012. The primary study outcome was culture conversion rate within 2 months of treatment. Results: Culture-positive pulmonary tuberculosis were diagnosed in 494 patients. 105 out of 499 patients (21%) had DM at the diagnosis. Among 105 patients with DM, 62 (59.5%) patients were treated with Metformin. Baseline characteristics including age, sex, the presence of cavity, underlying medical illness, and random glucose levels were not significantly different between metformin and non-metformin group. Patients with Metformin treatment had 1.5 times higher sputum culture conversion rate than those without Metformin treatment (P=0.041). For the recurrence within 1 year, Metformin did not show any significant effect and only cavity lesion significantly increased the recurrence. Conclusion: Metformin increased culture conversion rate at 2 months among TB patients with DM. It suggests that Metformin can enhance the effect of anti-TB treatment.