Hyperthyroidism is diagnosed when a patient is presented with typical symptoms, such as weight loss, palpitation, and hand tremor. However, early diagnosis is difficult in elders, because they have no typical symptoms, but only cardiovascular or muscular symptoms. In hyperthyroidism, there are often with neurologic changes, leading to various neurologic symptoms. Generalized or focal seizures are rarely reported in thyrotoxicosis and thyrotoxic crisis. Further, cases of hyperthyroidism presented as generalized seizure attack are extremely rare. We report a case of hyperthyroidism. A patient is presented at the hospital with mental change, as well as generalized seizure, who was finally diagnosed to have Graves' disease. A 56-year-old male was admitted to the hospital because of mental change with generalized seizure attack. Initial neurologic evaluations, including a brain magnetic resonance imaging and electroencephalogram proved to be normal. But, thyroid function test showed abnormal results. We diagnosed him as Graves' disease and prescribed anti-thyroid drug. Thereafter, there has been no recurrence of neurologic symptoms for 12 months.