Postoperative atrial fibrillation (AF) is the most frequent arrhythmia after cardiac surgery. It is associated with cerebrovascular accidents, hypotension, pulmonary oedema, longer hospital stays, increased cost of the procedure, and mortality. A total of 240 consecutive patients undergoing cardiac surgery in our centre between February 2010 and April 2010 were screened for eligibility. To be included in the study, patients needed to be referred for primary elective coronary artery and/or valve surgery, 18 years of age and in normal sinus rhythm. Exclusion criteria included prior coronary revascularization or heart valve surgery, emergency surgery, New York Heart Association class III or IV congestive heart failure, history of AF, hyperthyroidism, inflammatory diseases except coronary artery disease, infection, a left atrium size .70 mm, electrolyte imbalance, age ,18 years old, and ejection fraction ,0.25. The aim of this prospective, randomized, double-blind, placebo controlled study was to assess the effects of in the prevention of the postoperative AF. Patient take NAC from 48 before till 72 hours after surgery and control group take placebo . After completion of the surgical procedure, patients were admitted to the intensive care unit and when their haemodynamic and respiratory functions were stable, they were transferred to the wards. Rhythm was monitored continuously during the operation and during the first 2 postoperative days in the intensive care unit.