The domain structures of Bi3TiTaO9 (BTT) and SrBi2Ta2O9 (SBT) ceramics, which have the same crystalline structure but different fatigue properties, were studied by transmission electron microscopy. Antiphase boundaries and 90° domain walls in BTT ceramics were confirmed by the dark-field image taken with (031) superlattice reflection and selected area diffraction patterns, respectively. In contrast to SBT, 90° ferroelectric domains in BTT display a high, flat morphology and less density. Additionally, a type of planar defects normal to the c axis, which has not been found in SBT, was observed. The origin of different domain structures is discussed. The mechanism of fatigue behavior related to 90° domains is also proposed. © 2003 American Institute of Physics.