Gestational diabetes mellitus (GDM) is defined as glucose intolerance of variant severity with onset or first recognition during present pregnancy. Recently the prevalence of GDM in Korean has reported as 1.7~4.0%. Diabetic ketoacidosis is a serious metabolic complication of diabetes with high mortality if undetected. Its occurrence is very rare in gestational diabetes patients, but is harmful to fetal and maternal health. A 26 years-old pregnant woman was admitted at 37 weeks gestation because of progressive generalized weakness, anorexia and weight loss. Initial physical examination reveals that she had been dehydrated, and blood pressure 130/80 mmHg, pulse rate 100/min, respiratory rate 20/min, and body temperature was 36.9℃. Serum glucose was 545 mg/dL, pH 7.282, HCO₃^- 10.5 mmol/L, urine ketone 3+, urine glucose 2+ when initial laboratory work was done. She was treated with intravenous fluid and insulin under the impression of diabetic ketoacidosis. Her delivery was performed after 24 hours from admission because of suggestive fetal distress. After recovery, she is being treated with insulin at outpatient department. We experienced a appropriately treated case of diabetic ketoacidosis in pregnant woman with GDM, and report it with a literature review.