Postoperative atrial fibrillation is a major complication of cardiac surgery, which could lead to high morbidity and mortality, increase duration of hospital stay and increase the cost of treatment. New-onset atrial fibrillation after cardiac surgery is considered as a multifactorial phenomenon. Amiodarone, the most commonly used drug for cardioversion, is limited in atrial fibrillation after cardiac surgery due to side effects such as hypotension, bradycardia, and extracardiac side effects. Nifekalant is a novel class III antiarrhythmic agent with short onset time. It is a pure potassium channel blocker, which generally does not cause hypotension and bradycardia. There have been several trials that proven efficacy of nifekalant in converting persistent atrial fibrillation. For atrial fibrillation after cardiac surgery, the effectiveness and safety of nifekalant compared to amiodarone have not yet been reported. The investigators plan to perform a clinical trial comparing nifekalant to amiodarone in new-onset atrial fibrillation after cardiac surgery patients with a primary outcome of cardioversion at 4 hours. Secondary outcomes will follow cardioversion at 90 minutes and 24 hours, maintenance time of sinus rhythm within 24 hours, average time to conversion to sinus rhythm, rate of hypotension, length of ICU stay, length of hospital stay and hospital mortality.