Background : Mitral valve annulus dependent flutter is common arrhythmia after chronic atrial fibrillation( AF ) ablation. It is known that mitral valve isthmus line is the best way to prevent the flutter. But the isthmus area is very thick so actual isthmus block rate is not high. To get the annannul blosk block, we have tried anterior line and compared the results Methods and Results : 147 chronic persistent AF patients ( 106 males, mean age: 62짹13 years) who got AF ablation with open irrigated catheter, were enrolled. Bi-antral isolation, posterior or roof line and right carvotricuspid isthmus block were done for all patients. Mitral valve isthmus line was drawn in 111 patients ( group A ) and anterior line ( Figure 1 ) was made in 36 patients ( group B ). We defined ant. annulus block as more than 100 ms time delay from HRA to appendage in sinus. AF duration of group A and group B was 4.3짹1.7 vs 3.9짹1.4 years ( p< 0.142 ) and left atrial (LA) diameter was 43짹2.9 vs 45짹3.4 mm (p< 0.082). The mean follow up period was 17짹8 months The annulus block rate of group A and B was 28.8 vs 63.9 %, (p< 0.001). Atrial arrhythmia ( AF, flutter, atrial tachycardia(AT) ) recurrence of group A and group B was 37 (41/111 patients) vs 30.1 (11/36 patients) % ( p< 0.043), AF recurrence rate was 16.1 vs 19.6% ( p< 0.063 ), atypical la flutter and AT rate was 20.7 ( 23/111 patients), vs 13.8 ( 5/36 patients ) % ( p< 0.012). Among atypical flutter and AT recurrent patients, 8 of 23 in group A and 4 of 5 in group B were done redo ablation. In group A, 5 of 8 patients were proven mitral annulus dependent flutter, and in group B no patient showed annulus dependent flutter. During the ablation procedure, there was no difference of complications and no coronary artery injury was seen in both group Conclusions : Anterior line can be a better way to prevent mitral annulus dependent LA flutter than conventional mitral valve isthmus line.