BACKGROUND: Atrial fibrillation (AF) is the most common complication of cardiac surgery. Although it is managed easily, it can cause critical hemodynamic instabilities for intensive care patients. This observational study investigated the predictive power of P-wave dispersion (PWD) for the incidence of post cardiac surgery AF. METHODS: Among patients undergoing isolated coronary artery bypass grafting surgery (CABG), ۵۲ patients were selected randomly. Before the operation, ejection fraction, regional wall motion abnormality, and mitral regurgitation were determined by echocardiography. Angiographic data provided information about stenosed vessels. PWD was measured before and after CABG. The incidence of post-CABG AF was determined by rhythm monitoring. RESULTS: There were no significant differences in age, sex, stenosed vessels, maximum P-wave duration, the prevalence of hypertension, smoking, mitral regurgitation, and regional wall motion abnormality between post-CABG AF and non-AF groups (P > ۰.۰۵). The mean prevalence of diabetes mellitus in post-CABG AF group was more than non-AF group (P = ۰.۰۳۶). The mean ejection fraction in post-CABG AF group was lower than non-AF group (P < ۰.۰۰۵). The mean PWD in AF group vs. non-AF group before CABG was ۴۷.۵ vs. ۲۳.۷ ms. The mean values of post-surgical PWD in AF and non-AF groups were ۴۸.۱۰ and ۲۴.۴ ms, respectively. Before CABG, the mean ejection fraction value and minimum P-wave duration in AF group were lower than non-AF group (P < ۰.۰۰۵). A reverse relation was present between minimum P wave duration and PWD (P < ۰.۰۰۱). There was a negative association between high ejection fraction values and decreased PWD (P = ۰.۰۰۲). CONCLUSION: Our data suggested minimum P wave duration, PWD, and low ejection fraction are as good predictors of AF in patients undergoing isolated CABG. The absence of differences in age, sex, smoking, hypertension, mitral regurgitation, and regional wall motion abnormality in our study was in contrast with other reports. On the other hand, increased rate of post-CABG AF in our diabetic patients with lower ejection fraction supports other studies. Overall, minimum P wave duration, PWD, and low ejection fraction can be used for patient risk stratification of AF after CABG. Keywords: Atrial Fibrillation, Coronary Artery Bypass Grafting, P-Wave Dispersion, Predictor