Background Focal atrial tachycardia (AT) with two or more different activation sequences was regarded as multi-focal AT (MAT). However, a single focal AT arising from specific anatomic structure, such as crista terminalis (CT) or coronary sinus (CS) ostial musculature, can have multiple exits or interact each other, seemingly MAT. Methods and Results Out of 60 consecutive patients with focal AT undergoing radiofrequency catheter ablation (RFCA), 16 patients (26.7%, mean age 38.5짹16.8, male 50%) who demonstrated multi-sequences of AT arising from CT or CS were investigated. CT, CS, right atrial posteroseptum, and His recording area were mapped during spontaneous or induced ATs by burst pacing with isoproterenol infusion. RFCA was performed with point lesions targeting earliest activation of ATs. Results: The multi-sequence of ATs originated from CT or CS were more common in patients younger than 50 (33.3% vs. 9.52% those older than 50, p=0.02). None of them had structural heart disease and coexistent arrhythmias were present in 2 patients (1 left posteroseptal concealed bypass tract and 1 cavotricuspid isthmus-dependent atrial flutter). In 3 out of 16 patients, point ablation at a single focus (2 in CT, 1 in CSos) eliminated multiple activations of AT at once. Conclusion Focal AT with multiple activation sequences is relatively common phenomenon in AT arising from CT or CS ostium, masquerading MAT. The electrophysiologic interaction between CT and CS enables that a single focus from one site exits into another area or triggers each other. These findings have important implications for RFCA of focal AT with multiple activation sequences.