Background: Atrial fibrillation (AF) is one of the most common complications after cardiac surgeries. The incidence of postoperative AF has risen continuously over the past decades. AF is associated with lengthened hospital stays and risk of stroke. We sought to study the relationship between the pulmonary artery pressure (PAP) and the occurrence of AF after coronary artery bypass graft surgery (CABG). Methods: This prospective observational study was designed to assess the relationship between the PAP and the occurrence of post-CABG AF. Patients with chronic and paroxysmal AF before surgery were excluded. All the patients had complete evaluation via echocardiography, ECG, and laboratory testing. The patients were monitored for 3 days after surgery, and any tachycardia monitored as AF was noted. The study population was divided into 2 groups: with postoperative AF and without AF. Results: We ed 232 patients, 106 with AF and 126 with sinus rhythm. The results confirmed that the occurrence rate of AF after CABG was higher in the older patients (P ≤ 0.001). Both univariate and multivariate analyses showed a significant relationship between a higher occurrence rate of post-CABG AF and a higher PAP (mean value = 26.5 vs 20 mm Hg) in the patients (P ≤0.001 and P = 0.01, respectively). Conclusions: Although age has been the most important predictor for the occurrence of AF after CABG in the past and present studies, there are many other variables affecting its occurrence. Among the variables evaluated in this study, a higher PAP was a significant predictor for a higher occurrence rate of AF following CABG. (Iranian Heart Journal 2017; 17(4): 17-20)