ABSTRACTThe dielectric properties of (Ba0.92-xBix)(Ti1-yMny)O3 (BBTM)(x = 0.01–0.03, y = 0.008 or 0.016) ceramics were investigated. For bismuth content smaller than 0.02, BBTM appeared as a normal ferroelectric. For x = 0.02, there exist a clear diffuse phase transition character, associated with a broad permittivity peak vs. temperature without frequency dispersion. For x> 0.02, a relaxor behavior is obtained and frequency dispersion becomes more profound as the bismuth content increases. Atomic computer simulation has been used to predict the energy for different defect associations [], [] and [] which leads to random electrical fields and polar nanoregions (PNRs). The change from normal ferroelectrics to relaxors could be related to the size of PNRs.