Gestational diabetes mellitus is a major and prevalent pregnancy-related complication. Its identification and treatment improve perinatal outcomes. Aim is to assess the efficacy and short-term safety of Metformin versus Insulin in relation to maternal, fetal and neonatal outcomes in gestational diabetes mellitus after 20 weeks gestation. This is a prospective randomized study, carried out at the Diabetic Clinic of Nasser central hospital, from July 2020 to May 2021. The study included 85 pregnant women with gestational diabetes mellitus, were classified into 2 groups: Group (A) included 43 women received insulin and group (B) included 42 women received metformin. Fasting, random, postprandial blood glucose (PPBG), HbA1c, were determined. Primary outcome (diabetic control) and secondary outcome (maternal and neonatal outcome) were assessed. Regarding demographic characteristics, no significant difference was found. Regarding diabetic control, both Insulin and metformin achieved good comparable diabetic control. Similar results were found in HbA1C after 1, 2 and 3 months of treatment in both groups. The birth weight was significantly lower in Metformin group. While, the 5 minutes APGAR score was significantly higher in Metformin group. No significant differences were noticed regarding almost all neonatal outcome in terms of macrosomic baby, prematurity, shoulder dystocia, respiratory distress, neonatal jaundice, neonatal hypoglycemia and NICU admission. There was non-significant decrease in the percentage of macrosomic baby, neonatal hypoglycemia and NICU admission in the Metformin group. Metformin has efficacy as that of insulin in glycemic control of GDM and it may reduce the risk of some adverse neonatal outcomes.