Intoduction: Nonketotic hyperglycinemia (NKH) is one of rare inborn error metabolism disease and presents hypotonia, myoclonic seizure, hiccup and developmental delay mostly in neonates and infants. Pharmacological agents like sodium benzoate, dextromethorphan and diazepam decreasing glycin level, has been tried. However the long term outcome is poor. Case: A 4-month-old infant with intractable seizures was transferred to our hospital. He presented hypotonia at birth, and evaluated metabolic and genetic studies at other center in neonatal period. But at that time, the results were unremarkable. At one month of age, he showed recurrent seizure of tonic spasms, which were uncontrolled despite various antiepileptic drugs. He showed not only hypotonia, myoclonic seizure, but also characteristic “hiccups” and severe apnea requiring mechanical ventilation. We re-evaluated brain MRI, EEG, and the metabolic studies, that revealed CSF/serum glycin ratio of 0.14 (＞0.08) allowing the diagnosis of NKH with early myoclonic encephalopathy (EME). Later, molecular analysis confirmed the diagnosis of NKH, showing a homozygous mutation on GLDC gene(c.2258A＞C(p.His753Pro)(H753P)). After we started the treatment with sodium benzoate and diazepam, the glycin level was decreased, however his seizures was still intractable. We started the ketogenic diet and the result was favorable. Not only the frequency of seizure decreased, but also the hypotonia slightly improved. Conclusion: We suggest that ketogenic diet in combination with standard therapy in NKH with EME could be considered as a valuable therapeutic option to improve the motor and cognitive function.