Artrial fibrillation is the most common arrhythmia that occurs after coronary bypass grafting operation with the rate of 30%. Atrial fibrillation is associated with hemodynamic instability, strokes, and prolonged hospital stay. Pericardial effusion is a risk factor for atrial fibrillation after cardiac surgery, and it occurs commonly in the posterior area during the post-operative period. The aim of this prospective study was to demonstrate the effectiveness of posterior pericardiotomy in reducing the incidence of atrial fibrillation. This prospective randomized study was carried out on 425 patients undergoing a coronary artery bypass grafting in our clinic between August 2009 and February 2011. There were 276 male patients and 149 female patients. These patients were randomly divided into two groups; posterior pericardial incision was performed in 213 patients (pericardiotomy group), while any pericardial incision was not performed in 212 patients (control group). Atrial fibrillation occurred more frequently in control group (62 patients, 14.6%), compared to the pericardiotomy group (14 patients, 3.1%; p < 0.0001). The incidences of early pericardial effusion, late pericardial effusion, and tamponade were also significantly higher in control group. Moreover, posterior pericardiotomy was associated with the decreases in the duration of stay in hospital and intensive care unit. In fact, the total hospital costs were lower in the pericardiotomy group. In conclusion, posterior pericardiotomy is an effective and safe technique that reduces early pericardial effusion, atrial fibrillation, length of stay in hospital, and hospital costs after the coronary artery bypasses grafting.