The cardiovascular manifestations in hyperthyroidism are sinus tachycardia, paroxysmal supraventricular tachycardia, atrial flutter, atrial fibrillation, atrioventricular block, bundle branch block(especially right bundle branch block), angina pectoris, heart failure and cardiomyopathy. Of these, angina pectoris is commonly seen in hyperthyroidism with coronary artery disease and the potential mechanisms have been attributed to the increased metabolic demand and consequently increased cardiac work which result in the more demand of coronary blood flow than that can be delivered via a fixed atherosclerotic coronary artery stenosis. Hyperthyroidism associated anginas without underlying coronary artery stenosis have also been reported where the mechanism of these was suspected to be the coronary vasospasm. Ventricular fibrillation may occur in the thyrotoxic patients due to myocardial ischemia such as variant angina, but it is very rare in the condition without previous heart disease. A 30-year-old male was admitted to the hospital because of palpitation, weight loss and proptosis for the previous 3 months. There was no history of effort related chest pain, syncope, drug abuse or medical illnesses such as diabetes mellitus, hypertension. The laboratory results were, TSH: 0.38uU/mL(0.4~5,0 uU/mL), free T4: 8.9ng/dL(0.8~1.9ng/dL), TSH receptor antibody: 43.6%(-15~15%), antiTPO antibody: 5000 IU/mL(0~100 IU/mL). The initial EKG showed normal sinus rhythm. He was diagnosed as Graves disease with ophthalmopathy, class 3a and was put on propylthiouracil 200 mg po tid, propanolol 40 mg po tid and started solumedrol pulse therapy for the exophthalmos on the first day of admission. He was found to have generalized tonic seizure with apnea attack on second hospital day and twice thereafter. Ventricular fibrillation was documented at that time. DC cardioversion was performed with successful response. After the attack, he was treated as accelerated hyperthyroidism namely with increased dosage of propylthiouracil, dexamethasone and Lugols solution, The echocardiogram, treadmill test, ergonovine echocardiography, coronary angiography and electrophysiologic study disclosed no abnormalities. Further episodes of ventricular fibrillation didnt occur after being euthyroid state. In conclusion, we report a case of ventricular fibrillation associated with hyperthyroidism itself without underlying coronary artery disease with brief review of literatures.