The Koch's triangle of 110 human hearts (adults 70, children 40) has been dissected. The morphology, relationship and landmark of the atrioventricular node were observed and measured. Muscle bundles from the interatrial septum and coronary sinus orifice above and below are connected with the node. In the children the tendon of Todaro is usually tendinous, but in the adults, its posterior part is usually muscular. Deep in the triangle, there is a pyramidal space bounded by the left and right atrial walls and the top of the interventricular septum, in which there are vessels and nerves leading to the atrioventricular nodal area. According to their construction, the Koch's triangle is divided into five parts. The functional and surgical significance of these structures has been discussed.