Objective: The aim of this study is to compare the efficacy of metformin with that of insulin in treatment of gestational diabetes mellitus (GDM). Subjects & Methods: The study included 94 pregnant women who have been diagnosed as gestational diabetics at 25-33 weeks gestation with singleton pregnancy. They had fasting blood glucose (FBG) level ranging from 95-120 mg/dl or 2-hour postprandial blood glucose (PPBG) level ranging from 120-190 mg/dl. The exclusion criteria include pregnant women with preexisting DM and underlying diseases known to affect fetal growth or drug clearance. All patients were randomized to receive metformin (n=47) or insulin (n=47). Results: There were no significant differences between the two groups regarding maternal age, gravidity, parity, GA at time of diagnosis, GA at beginning of treatment, and BMI at time of diagnosis. Additionally, it was noticed that women in the metformin treated group reached sooner to the glucose targets and maternal weight gain was less in the metformin treated group. It was found that women who required supplemental insulin had higher BMI, earlier gestational age at the start of treatment and higher levels of FBG and 2 hours glucose level at time of diagnosis. Conclusion: Analysis of the results revealed that metformin was an effective medication for control of blood glucose in women with GDM who failed to achieve euglycemic with diet only.