A three dimensional computer model of human atrium is proposed to simulate atrial tarchycardia. This consists of the electrophysiology of human atrial cell model and the electric wave propagation in three dimensional atrial tissue. Electrophysiology of an atrial cell is numerically approximated using the previous human model proposed by Nygren et al. (1997). The three dimensional geometry of human atrium is discretized to a finite element mesh for the simulation of electric wave propagation. To characterize electrical activity in both atrial muscle and cardiac pacemaker regions, monodomain method is employed. The model equations of monodomain method were applied to a realistic geometry containing major topological features of both atria. The model is able to generate spontaneous activation of electrical impulses within the sinoatrial node which propagate across the atria. It is shown that the atrial tarchycardia is generated by an ectopic focus near pulmonary vein. To analyze the charateristics of the simulated atrial tarchycardia, the computed results on action potential tarnsient and pseudo-ECG pattern were also presented.