Objective: The aim of our study was determine whether aortic knob width (AKW) is associated with the development of atrial fibrillation (AF) after isolated coronary artery bypass surgery (CABG). Methods: In this retrospective observational cohort study, we evaluated 135 patients without hemodynamically significant valvular problems. AKW was measured on chest X-ray by digital system. Multiple logistic regression analysis was used to find independent associates of postop- erative AF (POAF). The diagnostic value of AKW was assessed using ROC analysis. Results: POAF occurred in 43 (31.8%) of all patients. The age, AKW, left atrial (LA) diameter and C-reactive protein (CRP) were significantly higher in patients with POAF than without POAF (67.2±8.6 vs 61.3±9.8 years, p=0.004; 45.6±5.8 vs 36.1±3.8 mm, p<0.001; 37.9±3.5 vs 35.8±3.1mm, p=0.002 and 10.6±8.5 vs 5.6±6.5 mg/L, p=0.001 respectively). Multiple logistic regression analysis demonstrated that AKW, LA diameter and CRP were independently associated with POAF (OR=4.527, 95% CI=1.315 -15.588, p=0.017; OR=2.834, 95% CI=1.091-7.360, p=0.032 and OR=1.300, 95% CI=1.038-1.628, p=0.022 respectively). ROC analysis has demonstrated that aortic knob of 36.5 mm constitutes the cut-off value for the occur- rence of POAF with 84.4% sensitivity and 64.6% specificity (AUC=0.84, 95% CI=0.75-0.94, p<0.001). Conclusion: We have demonstrated a significant association between the AKW and AF development after isolated CABG. PA chest radiography is a cheap and readily available clinical tool and it can be examined easily by every cardiovascular surgeons.