Background : The occurrence of atrial fibrillation after ablation of atrial flutter is clinically important. We investigated variables predicting this evolution in ablated patients without a previous atrial fibrillation history. Materials and Methods : Thirty-six patients (Male=28) who were diagnosed as atrial flutter without previous atrial fibrillation history were enrolled in this study. Group 1 (n=11) was defined as those who developed atrial fibrillation after atrial flutter ablation during 1 year follow-up. Group 2 (n=25) was defined as those who has not occurred atrial fibrillation during same follow-up term. Echocardiogram was performed to all patients. We measured left atrial size, left ventricle end diastolic and systolic dimension, ejection fraction and left atrial volume index before and after ablation of atrial flutter. The differences of each variables were compared and analyzed between two groups. Results : The preablation left ventricular ejection fraction (preLVEF) and postablation left ventricular ejection fraction (postLVEF) are $54{pm}14%$, $56{pm}13%$ in group 1 and $47{pm}16%$, $52{pm}13%$ in group 2. The differences between each two groups are statistically insignificant ($2.2{pm}1.5$ in group 1 vs $5.4{pm}9.8$ in group 2, p=0.53). The preablation left atrial size (preLA) and postablation left atrial size (postLA) are $40{pm}4mm$, $41{pm}4mm$ in group1 and $44{pm}8mm$, $41{pm}4mm$ in group 2. The atrial sizes of both groups were increased but, the differences of left atrial size between two groups before and after flutter ablation were statistically insignificant ($0.6{pm}0.9mm$ in group 1 vs $-3.8{pm}7.4mm$ in group 2, p=0.149). The left atrial volume index before flutter ablation was significantly reduced in group 1 than group 2 ($32{pm}10mm^3/m^2$, $35{pm}10mm^3/m^2$ in group 1 and $32{pm}10mm^3/m^2$, $29{pm}8mm^3/m^2$ in group 2, p<0.05). Conclusion : The difference between left atrial volume index before and after atrial flutter ablation is the robust predictor of occurrence of atrial fibrillation after atrial flutter ablation without previous atrial fibrillation.