Background The vein of Marshall (VOM) which exists along the peri-mitral isthmus (PMI) constitutes muscular connection between the coronary sinus (CS) and the left atrium (LA). Therefore, the anatomical variation of VOM would affect the outcome of linear ablation at PMI. Method and Results Forty one patients (35 male, 55.0짹9.6 years old) with non-paroxysmal AF (NPAF) who underwent PMI block were included. After bilateral pulmonary vein (PV) antral ablation, linear ablation at PMI was performed (irrigation tip catheter, 30W, 25mL/min) along the VOM after CS venogram. Complete bidirectional block was verified by differential pacing at proximal CS and LA appendage. If we could not achieve PMI block with endocardial ablation, ablation underwent inside CS (25W, 20mL/min). CS venogram and PV computed tomography (CT) were analyzed and correlated with the clinical outcome. Results: 1. PMI block was achieved in 22.2% after endocardial LA ablation and 68.8% after additional inside CS ablation (p<0.01). 2. VOM was clearly visible in 78.0% by occlusive CS venograms. In patients with failed PMI block, VOM length was longer (16.0짹13.1 mm vs. 14.5짹5.1 mm on LAO 35째, p<0.01; 24.0짹16.4 mm vs. 15.5짹9.5 mm on RAO 35째, p<0.001) and the angle between CS and VOM was shallower (42.7짹19.1째 vs. 58.3짹35.7째, p<0.01 on RAO35째) than those with successful PMI block. 3. On the PV CT image, LA volume (132.8짹29.8 mL vs. 147.0짹60.5 mL, p=NS) and PMI length (108.3짹16.5 mm vs. 110.6짹14.8 mm, p=NS) were not different between patients with failed PMI block and successful PMI block. Conclusion In patients with longer VOM with shallow its angle between CS, the achievement of PMI block was difficult during catheter ablation of NPAF. Identification of VOM can guide efficient PMI block.