As the electrical source of hearts, sino-atrial node (SAN) plays a significant role in rhythmic firing. The aim of this study is to determine effects of atrium on the electrical activity of SAN. Based on a mathematical model considering central and peripheral SAN cells as well as an atrial model, one-dimensional fiber with a gradual heterogeneity of SANC was developed. The forth-order Runge-Kutta and three-point centered difference methods were used to integrate the conduction model. The results demonstrated that as a load of SAN, atrium decreased the spontaneous sinus rate casing a shift of leading pacemaker site from the periphery to the center. If the ectopic beat produced by a premature stimulation was late after the end of the atrial refractory, the pause following it was compensatory. Otherwise, the pause was non-compensatory. In this case, the peripheral SANC excited earlier than the central SANC. During multiple premature beats, at short intervals the propagation could block in SAN due to a long refractory period of the action potential close to the central SANC. Leading pacemaker site shift was observed during the following several spontaneous SANC excitation. Therefore, the existence of atrium is suggested affecting not only sinus rates, but also the mode of electrical propagation and the site of the earliest excitation.