Introduction: Prompt diagnosis and management of atrial tachyarrhythmias during catheter ablation (CA) of atrial fibrillation (AF) are still challenging. Methods: 1. Periannular activation time (PAT) around the mitral annulus (MA) was measured by the activation time difference between the most proximal (CS 9,10) and distal (CS 1,2) poles of the coronary sinus (CS) electrodes (inter-electrode distance, 2-10-2-10mm). 2. Entrainment pacing (EP) was performed around the MA or cavo-tricuspid isthmus (CTI), and post-pacing interval (PPI) was measured. Results: 1. In 39 patients with AF undergoing CA of AF, 54 regular atrial tachyarrhythmias were induced or converted from AF. 2. Mechanisms of tachycardias were macroreentry around the MA (PM-AFL, N=22), roof-dependent atrial flutter (Roof-AFL, N=8), CTI-dependent AFL (CTI-AFL, N=9) or atrial tachycardia (AT, N=15). 3. When PAT ��� 45ms, MA activation sequence was sequential, either proximal to distal or distal to proximal. When PAT < 45ms, MA activation sequence was non-sequential with converging or diverging patterns. PAT ��� 45ms was a highly specific indicator to rule out Roof-AFL from PM-AFL (PM). 4. When combined with PPI data from MA and CTI, the algorithm (table) successfully diagnosed PM-AFL (20/22, sensitivity 91%, specificity 91%), Roof-AFL (7/8, sensitivity 88%, specificity 91%), CTI-AFL (7/9, sensitivity 78%, specificity 100%) and AT (12/15, sensitivity 80%, specificity 92%). Conclusions: Diagnostic algorithm combining PAT and entrainment pacing provides a rapid assessment of the mechanism of atrial tachyarrhythmias during catheter ablation of atrial fibrillation.