Introduction: When we performed radiofrequency catheter ablation for atrial fibrillation(AF), we usually put large-sized lasso catheter at both pulmonary antral area and tried to isolate the antrum electrically. We defined the complete antral isolation like that any potential at both upper and lower pulmonary veins was not found after antral isolation. We elucidated the effect of complete isolation at antral site in pulmonary vein on AF recurrence. Subject and Methods: 132 patients (M:F=88:44, mean age:57.7짹10.3) who underwent isolation of both antral,roof and mitral isthmus site with technique of CARTO Merge and large-sized lasso were enrolled. We divided the patients into four groups by the achievement of complete isolation: Group 1(n=15) was achieved complete isolation at both pulmonary antrum, Group 2 (n=20) at only left side complete antral isolation, Group 3 (n=36) at only right side complete antral isolation, and Group 4 (n=61) without complete isolation in either of the both pulmonary antrum. We compared the recurrence rates among the four groups.Results: The recurrence rates for AF among the group 1,2,3 and 4 were different significantly[0%, 0%, 8.3%, 11.5%(p=0.028), respectively]. The recurrence of atrial flutter or atrial tachycardia among the four groups showed the trend of decrement [6.7%,15%,13.9%, 16.4%(p=0.068)]. Complete isolation at left side compared to right might be more closely related with AF recurrence (0% vs 8.3%, p=0.002).Conclusion: Complete pulmonary antral isolation is important for preventing AF recurrence with technique of CARTO Merge and large-sized lasso. Especially, left side is more significant than right side.