A 65-years-old male patient was hospitalised with acute inferior myocardial infarction and sinus bradycardia. The patient was diagnosed with coronary artery disease accompanied by severe tricuspid insufficiency. Coronary artery bypass grafting concomitant with De Vega's tricuspid annuloplasty operation was performed. During the operation, a diverticulum shaped mass in the right atrial appendage was excised. Pathological examination revealed that the mass was a thrombus. Right atrial thrombus is less frequently seen than left atrial thrombus. In addition, it rarely develops in a patient in sinus rhythm. Although right atrial thrombus is rarely seen, it may cause severe thromboembolic complications. The possibility of right atrial thrombus should be kept in mind in the patients undergoing coronary artery bypass surgery after inferior myocardial infarction. We think that when right atrial thrombus is detected during the operation, it should be removed to avoid possible pulmonary embolism.