Common-type atrial flutter (AFL) is a type of atrial tachyarrhythmia with counterclockwise rotation around the tricuspid annulus within the right atrium (RA). It was recently reported that the electrogram voltage reduction observed in the RA was involved in the development of AFL. However, the relationship between the low voltage areas and conduction velocity during AFL has not been fully described. In this study, patients with AFL (n = 17) and without AFL (n = 4) were examined using an electro-anatomical mapping system. The patients with AFL were divided into 2 groups; AFL group (n = 8) and coronary sinus ostium (CSO) group (n = 9). The AFL group was defined as exhibiting the maintenance of AFL and the CSO group sinus rhythm before the catheter ablation. The electrogram voltages of each area in the RA (septum, and posterior and lateral walls), conduction velocity during AFL and transverse and longitudinal conduction velocities were evaluated. In the septum, the mean electrogram voltage was significantly lower in the AFL and CSO groups than in the group without AFL. Moreover, the conduction velocity during AFL was significantly slower in the septum, and both the septal transverse and longitudinal conduction velocities were significantly slower in the AFL and CSO groups than in the group without AFL. In conclusion, these findings suggest that both the slower conduction velocities and lower voltage in the RA septum may be involved in the development of AFL. Thus, ablation of the RA septum may represent a therapeutic approach of AFL.