Fiber orientation anisotropy in the human atria influences the success of electrical cardioversion. This computational study quantifies the effectiveness of a feedback controlled cardioversion method [1] using a novel cardiac simulation environment, BeatBox [2]. A biophysical human atrial cell model [3] was adopted to simulate Control and chronic atrial fibrillation (CAF) action potentials (APs). Electrophysiological alterations due to CAF were simulated as described previously [4]. The cell model was combined with human atrial geometry with fiber orientation to construct a 3D realistic human atrial model. Scroll waves were initiated using a phase distribution method and allowed to evolve for 10 s. The cardioversion parameters [1] of threshold stimulus and low energy stimulation strength required for scroll wave termination were evaluated. Scroll waves were initiated at two sites to elicit the effects of scroll wave location. The obtained results were compared a previous isotropic case study [5]. CAF reduced the AP from 306 ms to 120 ms (Fig 1, A). Scroll waves were initiated at two locations and the electrical activity registered at a point registration electrode (Fig 1, B). Under CAF isotropic conditions, the scroll waves became pinned (Fig 2, A). In contrast, anisotropy caused the scroll wave to degenerate into multiple scrolls with each scroll evolving erratically or pinning to anatomical defects (Fig 2, A). Recurrence cycle maps (Fig. 2, B) show that under CAF isotropy leads to a monomorphic tachycardia while with anisotropy gives rise to a polymorphic tachycardia. The single shock stimulation threshold to eliminate CAF scroll waves was found to be 4.5 pA/pF. For CAF scroll waves initiated at location L1 (Fig 1, B), a series of small amplitude global stimuli of strength 0.4 pA/pF were sufficient in terminating the scroll waves. However CAF scroll waves initiated at L2 required a minimum stimulation strength of 2 pA/pF for termination within 10 s. In all CAF simulations, the localized pacing at the registration electrode was approximately 8 Hz. Inherent atrial anisotropy plays an important role in atrial electrical properties. The low energy stimulus strength has a strong correlation to the scroll wave location. The low energy stimulation strength was always lower than the threshold stimulus. Anisotropy aggravates CAF and leads to high frequency erratic propagations. The efficacy of cardioversion is significantly affected by anatomical defects as well as anisotropy. The novel multifunction simulation package, BeatBox, is an ideal simulation environment that facilitates such extensive large scale simulation studies.