Objective: Atrial fibrillation (AF) is the most common arhythmia type among other arythmias. In this study, we aimed to search the effect of left atrium diameter in AF treatment with radiofrequency ablation procedure. Methodology: Preoperative left atrium diameter and postoperative sinus rhythm restoration was prospectively studied in 84 patients who had undergone valve surgery and radiofrequency (RF) ablation procedure in Department of Cardiovascular Surgery from January 2004 to March 2009. The patients were classified in two groups. Group-I consisted of the patients with left atrial diameter less than 5 cm, and Group II consisted of the ones whose left atrial size was more than 5cm. Normal sinus rhytm restoration was followed by electrocardiography (ECG). ECG monitoring was done in early postoperative period, 12th, 24th and 36th monthes following the operation. Results: In early post-operative period normal sinus rhythm (NSR) was recorded in 34 patients (Group I: 20, Group II: 14) in which unipolar RFA was performed. Supraventricular tachycardia (SVT) was observed in 20 patients (Group I: 10, Group II: 10) and NSR was restored with medical treatment in these patients. AF was permanent in 26 patients (Group I: 8, Group II: 18) and nodal rhythm was observed in 4 patients (Group I: 2, Group II: 2). The ECG monitoring was done in the patients in early postoperative period at 12th, 24th and 36th monthes. Datas were evaluated with statistical studies, too. Conclusion: Radiofrequency (RF) ablation is a feasible, efficient and safe method for the treatment for Atrial fibrillation (AF). In our study we found that left atrial diameter is an important factor in restoring sinus rhythm. However, AF treatment with extended left atrium is more difficult.