The ultra rapid potassium current (IKur) is an attractive pharmacological target in atrial fibrillation (AF) management due to its atrial specific nature. An experimentally based 78% IKur reduction was incorporated into a human atrial action potential (AP) model under sinus rhythm (SR) and atrial fibrillation (AF) conditions. Its effects on cell and tissue level electrical activity were simulated. IKur block reduced AP duration (APD) and effective refractory period (ERP) under SR conditions, but prolonged APD and ERP under AF conditions. IKur block modulated tissue's ability to sustain high pacing rate conduction under SR and AF conditions. Vulnerability window (VW) was augmented under SR, and reduced under AF conditions. IKur block did not effect on re-entrant waves in 2D and 3D simulations. Simulations show pro-arrhythmic effects in SR, but anti-arrhythmic effects in AF case due to IKur block.