Ligament of Marshall (LOM) can be a source of atrial fibrillation (AF). However, successful cannulation and mapping of LOM via coronary sinus (CS) can be achieved in only 37% of patients. We hypothesize that epicardial mapping of LOM can be performed with a higher success rate during AF ablation. Percutaneous sub-xyphoid pericardial puncture was used to insert a duodecapolar mapping catheter into the pericardial space and positioned near the vein of Marshall (VOM), which was identified by the occlusive coronary sinus venograms. A total of 49 patients (31 M, 66±9 YO) with either paroxysmal (N=31) or persistent (N=18) AF were studied. The procedure time was 172 ± 56 minutes . Successful epicardial mapping of LOM was achieved in 43 patients (88%), including 1 of the 4 patients with previous coronary artery bypass surgery. In sinus rhythm, the LOM activated from coronary sinus (proximal) to LA or PV (distal) in 21 of the 43 patients, and from distal to proximal in the remaining 22 patients. These findings suggest that roughly half of the LOM connected directly to the LA and left PV, resulting in distal to proximal electrical propagation. The LOM pacing (threshold 1.6±1.1 mA) showed that the LOM directly connects to both left PV and the coronary sinus (dual connections) in 18 patients. Among them, ablation inside left PV trunk (N=15) or more selectively in the left superior PV (N=3) and both left superior and left inferior PV (N=1) eliminated the connection between LOM and PVs. Four patients (8%) had AF originating from the mid or distal LOM. The electrograms showed drivers within the LOM during AF. The AF ablation was successful in 35 patients (72%) with 1.6±0.9 months of follow up. The complications included pericardial bleeding in one patient with previous surgery and post procedure pericarditis in 16 patients. We conclude that epicardial mapping of the LOM can be performed with a high success rate in patients without prior open heart surgery. LOM can be a source of rapid activations in both paroxysmal and persistent AF. There may be multiple electroanatomical connections between LOM and the left PVs, and that these connections can be ablated with endocardial approach guided by an epicardial mapping catheter.