Comparative study of box-lesion set and past single ablation lesion set were made in 48 patient who underwent the Cox maze IV procedure for atrial fibrillation (Af) from October 2004 through October 2009. Before May 2008, the pulmonary veins were connected with single ablation lesion set (n = 20), where as a box-lesion set was performed (n = 28) after that. Their mean age was 68.6 years (age range 49-87 years). Freedom from Af recurrence was greater than 80% in both groups and there was no significant difference between the 2 groups. When left atrial diameter was larger, freedom from fibrillation recurrence was lower in both groups. In conclusion, the Cox maze IV procedure produces good surgical outocomes. More study is necessary to evaluate the effectiveness of the box-lesion set.