Introduction: Neonatal hypoglycemia is the most common metabolic disorder in the infancy period and associated with a high risk of neurologic handicap and epilepsy. Congenital hyperinsulinism is a rare genetic disorder characterized by excessive insulin secretion and severe hypoglycemia caused by mutation in several genes such as ABCC8 and KCNJ11. We report a baby of West syndrome with congenital hyperinsulinemic hypoglycemia caused by mutation in ABCC8 gene. Case: A 6-month-old infant was born at 38 weeks gestation with a birth weight of 3.9 kg who was admitted to our institution because of hypoglycemia (40 mg/dL) at the first day after birth. The laboratory finding showed elevated level of insulin (36.75 uU/mL) and the mutation in ABCC8 gene detected by genetic analysis. Blood sugar level reached to normal range after using diazoxide since 4 weeks of age. Despite resolution of hypoglycemia, she presented with focal clonic seizure on 5 weeks of age and the seizure responded to phenobarbital and phenytoin. At 6 months of age, however, the seizure pattern changed to infantile spasms and the electroencephalography revealed the findings of hypsarrhythmia. Fortunately, her seizures were stopped 3 months later since the start of the antiepileptic drugs, and the findings of hypsarrhythmia were disappeared. Conclusion: Neonatal hypoglycemic brain injury is associated with subsequent development of early-onset epileptic encephalopathy, such as West syndrome. Early diagnosis and treatment of hypoglycemia is crucial in order to prevent neurological damage in infant period.