We report a 12 year-old girl patient with hypothalmic hamartoma presented a gelastic seizure for 11 years. On magnetic resonance images, a 10.5 mm-sized mass was detected that was originated from the tuber cinereum into the third ventricle. This mass was isosignal intensity on T1WI, slightly high signal intensity on T2WI and not enhanced with Gadolinium. On interictal EEG, the frequent spike or polyspike dischage was recorded from the left fronto-central area. On ictal EEG, the suspicious polyspike dischages followed by spike and wave discharge were recorded from the left or both fronto-central or fronto-centro-temporal area. On 18-FDG PET (18-fluorodeoxyglucose positron emission tomography), the glucose metabolism was decresed on the mass. We treated this lesion using gamma knife radiosurgery (18 Gy at 50% margin, 508.2 mm3 volume). After 6 months, the frequency of gelastic seizure was decreased.