Background: the use of anti-diabetic drugs to control gestational diabetes was controversial. Some studies suggested a possible link between the use of oral anti-diabetics and fetal anomalies, fetal macrosomia and neonatal hypoglycemia whereas others have demonstrated no such relationship. Metformin is a biguanide hypoglycemic agent that reduces hepatic gluconeogenesis and increases peripheral insulin sensitivity. Although it crosses placenta, metformin appears to be safe in pregnancy. Aim of the Work: this study aimed to assess the efficacy of metformin in controlling maternal blood glucose level compared to insulin in women with GDM. Patients and Methods: this randomized controlled trial was conducted on 116 patients with GDM recruited from the outpatient clinic of Ain Shams University Maternity Hospital (ASMH), Cairo, Egypt from February, 2016 to January, 2017. Results: macrosomic baby was significantly less frequent among metformin group than among insulin group (p= 0.047). Conclusion: metformin has efficacy as that of insulin in glycemic control of GDM and has the following beneficial effects: reduction the rate of shoulder dystocia, reduction the rate of cesarean section and reduction the rate of macrosomia more than insulin. Recommendations: metformin is recommended as an alternative to insulin therapy in control of blood glucose in patient with GDM when diet therapy and exercise fail to reduce blood glucose values sufficiently. The time for metformin as an alternative treatment to insulin has come; however, it should be prescribed after careful consideration of these patient characteristics to minimize the need for supplemental insulin.