The characteristics of cation diffusion with many-body effects have been discussed in Ag beta-alumina by MD calculation. It was found that the Ag-Ag repulsion changes the dynamics of the Ag ions from a random hopping to a cooperative motion. When the repulsive energy of Ag-Ag pair becomes strong, the Ag ions take the configuration to separate the micro domains which belong to BR or aBR sites, and the ionic diffusion becomes difficult by the additional energy required for the structural relaxation of its partial ordered region. It shows that the ionic radius of the mobile ion plays an important role to the fast ionic diffusion. (C) 2012 Elsevier By. All rights reserved.