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Background Ablation devices are being utilized in surgical procedures treating atrial fibrillation. Few studies seek to define a cut-off value for left atrial diameter size beyond which risks of procedure outweigh chance of sinus recovery. This study aims to identify a cut-off value for pre-operative left atrial diameter to assess the efficacy of surgical radio-frequency ablation for treatment of chronic atrial fibrillation in patients undergoing mitral valve surgery. Methods A prospective 6-month follow-up cohort study was done, in which 40 patients were recruited during the period from May 2016 till April 2018. All patients had rheumatic mitral valve disease and permanent atrial fibrillation. Receiver operator characteristic (ROC) curves were used to calculate the area under the curve and cut-off value for left atrial diameter. The efficacy of the overall survival time was estimated using the Kaplan-Meier method. Results Pre-operatively left atrial diameter of > 59 mm was significantly associated with decrease in the possibility of reverting to sinus rhythm after surgery (OR 0.292, p-value = 0.001). The cut-off value for left atrial diameter was 59mm (sensitivity = 93.3%, specificity = 96.1%). Kaplan-Meier survival estimates were 175.07 days (156.3 - 193.9) for patients with left atrial diameter < 59mm and 62.64 days (26.6-98.7) for patients with left atrial diameter > 59mm. Conclusion In patients undergoing mitral valve surgery, higher degrees of ablation success are associated with smaller left atrial diameter in terms of conservation of the sinus rhythm post operatively.