Heart is easy to be affected by the abnormal thyroid function because cardiac muscle cells have many thyroid hormone receptors. In addition, thyroid hormone goes higher sensitivity to sympathetic nerve as it increases the number of myocardial beta receptor. Therefore, when the thyroid hormone is excessive value, tachycardia and atrial fibrillation may occur regardless of the cause. The atrial fibrillation in Graves' disease can expect spontaneous reversion to sinus rhythm when the hyperthyroidism is controlled. However, cardioversion is indicated for patients who have not returned to sinus rhythm for at least 3 months after the hyperthyroidism is controlled. In this paper, we report the relationship between hyperthyroidism and heart, and the treatment of atrial fibrillation in Graves' disease.