Abstract Objective: To evaluate the factors affecting the success of metformin in management of GDM. Methods: A prospective cohort study was done for 94 patients diagnosed with GDM from April 2019 to March 2020 who needed pharmacological treatments in addition to diet and lifestyle modification. Cases treated with metformin monotherapy were compared with others who needed insulin in addition to metformin for glycemic control. Patient characteristics, glycemic control data and neonatal outcome were evaluated. Univariate and multivariate analysis was done to find the independent factors affecting the success of metformin. Results: Of 94 patients with GDM who needed pharmacological treatment, 73 (77.6%) used only metformin for treatment of GDM and 21 (22.4%) needed insulin to be added to metformin. Multivariate analysis revealed that BMI was the only significant factor that affects the success of metformin alone in control of GDM (p = 0.03). ROC curve of BMI showed that the cut off value of the highest sensitivity is 32.1 above which metformin monotherapy failed to control GDM. Conclusion: Metformin can be considered as a safe and effective drug for treatment of GDM. Obesity was found as a predictive factor for failure of metformin monotherapy.