It was noticed long time ago that the dielectric permittivity of BaTiO3 single crystals in the rhombohedral phase varies from observation to observation, but the microstructure mechanism behind this phenomenon remained unclear. We show that the variations in susceptibility can be related to the variable domain structure introduced by different pre-treatments at higher temperatures: Electrical depoling at room temperature leads to nano-sized domains and ultrahigh susceptibility in rhombohedral phase, whereas thermal annealing at 450 degrees C results in micro-sized domains and relatively lower dielectric permittivity. The influence of the pre-treatments on domains evolution is tentatively rationalized with a stress accommodation mechanism. The ultrahigh dielectric permittivity could be ascribed to the by-produced 180 degrees-domain walls in the ultrafine domain structure. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3605494]