Objective To observe the structural characteristic of Marshall ligament and coronary sinus of 40 dog hearts. Methods The Marshall ligament and coronary sinus of 40 dog hearts fixed with 10% formaldehyde were observed in gross anatomy. Results The Marshall ligament begins at the right external side edge of left atrium appendage, via the left inferior pulmonary vein to the inner surface of pericardium. The length and the width of Marshall ligament of dog hearts were (2.61?0.95) cm, (0.18?0.04) cm respectively. The angle between Marshall ligament and the coronary sinus was (116?11), and the shortest length between Marshall ligament and left atrium was (0.58?0.52) cm. The length and the width of coronary sinus was (2.84?0.85) cm, (0.28?0.07) cm respectively. There are lots of cardiac fibers of ring (47.5%), vertical (37.5%) and diagonal (15%) shape on the surface of coronary sinus. Conclusion A lot of muscle fibers attaching to the surface of Marshall ligament and coronary sinus in dog hearts are abnormal atrial channels. Dog is the favorable animal for research of focal atrial fibrillation.