We report a 51-year-old man who was admitted to our hospital due to repeated episodes of syncope associated with generalized myasthenic symptoms. Due to myasthenic symptoms with the presence of anti-AchR antoantibody, he was diagnosed as myasthenia gravis (MG) associated with thymoma. However, Holter ECG showed long pause with maximum R-R interval of 3.8 seconds and paroxysmal atrial fibrillation, indicating the diagnosis of sick sinus syndrome. After pace maker implantation and combination therapy with thymomectomy and steroid administration, no arrhythmia in repeated Holter ECG was found. In addition, an anti-kv1.4 antibody was positive in our case. The involvement of cardiomyopathy in patients with MG has been reported, including the association with sudden death. The anti-kv1.4 antibody was recently identified in cases of myasthenia gravis associated with cardiomyositis. After treatments, no arrhythmia was found in our case. Although the cardiomyopathy was not diagnosed in our case because of lacking of histological confirmation, clinical course associated with positive anti-kv1.4 antibody suggested that the cause of syncope might be immune-related cardiomyopathy. To prevent fatal complication of arrhythmia, appropriate examination and therapy against cardiomyopathy associated with myasthenia gravis should be considered.