Type 1 atrial flutter (AFL) is a macroreentrant tachycardia in the right atrium; the anterior barrier of the common AFL circuit is located at the tricuspid annulus (TA) and the posterior border is functional line of block at the posteromedial (sinus venosa region) right atrium. The upper turnover site of the wave front is mainly located at the anterior to the superior vena cava. Conduction property across the posteromedial (sinus venosa region) right atrium in patients with and without AFL is different. i.e., functional conduction block occurs at the lower pacing rate in patients with AFL, and the majority of patients with chronic AFL demonstrate conduction block across the posteromedial (sinus venosa region) right atrium even during sinus rhythm. Catheter ablation therapy for AFL is creation of linear lesion between tricuspid annulus and inferior vena cava. Changes in activation sequence around the tricuspid annulus are used to confirm bidirectional block. However, it is difficult to demonstrate the bidirectional block in the presence of transverse conduction around the inferior vena cava. In such a case, bidirectional block should be confirmed by differential pacing or 3-dimensional mapping system.