Introduction: We evaluated the anatomical structure of CTI, which is frequent target during atrial flutter ablation, using cardiac CT to provide systematic information for the ablation procedure. Methods: Cardiac CT scans of 102 consecutive patients with structurally normal heart and sinus rhythm were evaluated. A long-axis image was reconstructed according to the line from the center of tricuspid valve to the center of inferior vena cava (TV-IVC line) in an axial image. We measured dimensions consisting of the CTI and the depth of right coronary artery (RCA). The total length of CTI was defined as the sum of the length of vestibular (A) and pouch area (B). Results: The axis angle of TV-IVC line was LAO 23 짹 11 degree. CTI morphology was classified into three types: pouch (65%), concave (27%) and flat (8%) types. There was no significant difference between the total length of CTI (pouch vs. concave vs. flat type = 23 짹 5 vs. 23 짹 7 vs. 24 짹 5 mm, p>0.05). Concave type had significantly deeper depth (d) of CTI (6 짹 2 mm) than pouch type (5 짹 2 mm, p=0.049). The mean depth of RCA was 2 짹 1 mm, and no significant difference was shown among three types. Conclusion: According to the average value, LAO 23-degree and RAO 67-degree images would be appropriate for CTI ablation procedure. More than 90% of CTI���s are not flat, and this fact should be considered during ablation. The anatomical information obtained in this study could provide essential information for a specialized catheter design for CTI ablation.