Traditionally, reoperations for mitral valve replacement are carried out under cardioplegic arrest with cross-clamping of the ascending aorta via a median sternotomy. In this case, the mitral valve replacement operation was performed with an on-pump beating heart technique without cross-clamping the aorta and via a right thoracotomy because of diffuse adhesions around the ascending aortic tube graft. A 44-year-old male patient had undergone a Bentall operation via a median sternotomy for annulo-aortic ectasia 3 years ago. He was admitted to the hospital complaining of palpitation and dyspnea. Transthoracic echocardiography revealed 4th degree mitral insufficiency. Mitral valve replacement was carried out through a right thoracotomy using an on-pump beating heart technique without cross clamping the aorta. In conclusion, mitral valve replacement with an on-pump beating heart technique via a right thoracotomy offers a safe approach when excessive dissection is required to place a cross-clamp to the ascending aorta.