A 54-year-old male patient with squamous cell carcinoma of lung was scheduled for right lower lobectomy. He had suffered from diabetes mellitus during 8 years, but his preoperative electrocardiogram was normal sinus rhythm. During right thoracotomy, 2 times of unstable paroxysmal supraventricular tachycardia (PSVT) appeared but returned to normal sinus rhythm by carotid sinus massage. But, third unstable PSVT was not controlled by adenosine and anterior-lateral paddle positioned biphasic cardioversion. We changed anterior-lateral paddle position to anterior-posterior paddle position. And PSVT terminated abruptly and changed to sinus rhythm. So we present a case of unstable PSVT during right thoracotomy and treated with anterior-posterior paddle positioned biphasic cardioversion.