Background: Atrial fibrillation (AF) can occur even after correction of mitral valve pathology in patients who are undergoing mitral valve surgery. However, factors associated with occurrence of AF after mitral valve (MV) surgery are unknown. The aim of this study was to investigate the factors determining occurrence of AF after MV surgery in patients who were undergoing MV surgery and sinus rhythm preoperatively. Methods: Four hundred fouty two-patients (190 men, mean age 46 짹 12) who were undergoing MV surgery and sinus rhythm were investigated. Transthoracic echocardiography was performed before and after MV surgery. Pre-and post operative left atrial volume index (LAVI) measured and percentage change of LAVI was calculated. Results: AF occurred in 81 (18%) patients even after successful MV surgery and preoperative sinus rhythm. It was more common in older age, lower pre-and post-operative left ventricular ejection fraction (LVEF), larger post-operative LA size and lesser degree of reduction in LA size after surgery. In multiple regression analysis, post-operative LAVI was independent predictor for occurrence of AF (Cut-off value: 38.7 ml/m2, sensitivity: 79%, AUC: 0.762, SE: 0.051, p < .001). Conclusion: Post-operative AF is not uncommon even after successful MV surgery despite pre-operative sinus rhythm. Larger post-operative LAVI was independent predictor for occurrence of AF.