Background This pilot study aimed to evaluate the effectiveness of posterior left atrial wall plication (T-plasty) in patients with persistent atrial fibrillation (AF) (> 7 days) undergoing mitral valve surgery. Materials and Methods A total of 60 patients who were scheduled for mitral valve replacement were randomly allocated into two groups: one would receive (Group 1; n = 32, mean age; 49.37 ± 9.00) and one would not receive (Group 2; n = 28; mean age 48.64 ± 8.6) left atrial size reduction using T-plasty technique. Patients with a clear indication for combined procedures other than tricuspid valve disease, aortic valve disease, and coronary artery stenosis were not included. Follow-up was performed at 6th, 12th, and 18th months after the operation. Results After the operation, 21 patients (65.6%) in Group 1 and 13 patients (46.4%) in Group 2 regained sinus rhythm (p = 0.13). Mortality did not occur. AF recurrence rates were not significantly different between the groups in three follow-ups. Restoration of sinus rhythm was significantly more common in Group 1 patients than in Group 2 patients during follow-up. Patients in Group 1 had lower left atrial volume indexes than those in Group 2 at the 6th and 12th months, whereas the difference at the 18th month was at the limit of significance. Conclusion We achieved satisfactory results using the T-plasty technique for left atrial size reduction in terms of mid-term restoration and preservation of normal sinus rhythm in patients undergoing mitral valve surgery. Further study may be justified to reveal the prognostic importance of the technique described herein.