Background: Sometimes, Prolonged procedure is needed in the ablation of the typical atrial flutter. It depends on isthmus length, thickness and muscle mass etc. We studied the parameters that could predict the length of isthmus in surface electrocardiogram. Methods: Eleven patients (8 male; 53짹13 years) who had typical atrial flutter and underwent cavotricuspid isthmus ablation guided by electroanatomical mapping were enrolled. The isthmus length was measured from inferior tricuspid annulus to inferior vena cava in electroanatomical map. The flutter cycle length and the amplitude of the F wave(peak to peak) in leads I, II, III, aVL, aVF, V1 were measured. Results: The average length of isthmus was 20.18짹5.75 mm. The length of isthmus, negatively correlated with the tachycardia cycle length and the amplitude of the F wave in the lead I(0.034, 0.031 respectively). Other results are in the table: Conclusion: The length of the isthmus could be predicted by tachycardia cycle length and the amplitude of F wave in lead I. It could help to make out the ablation strategy in the typical atrial flutter.