Microstructures of ordered Ba(Cd1/3Ta2/3)O-3 perovskite dielectric ceramics with and without a boron additive have been observed. by atomic resolution transmission electron microscopy (TEM). The selected area electron diffraction and lattice image show-a well-ordered structure with hexagonal symmetry (lattice constants of a similar to 5.8 and c similar to 7.1 angstrom) in the ordered Ba(Cd-1/3 Ta-2/3)O-3 with a boron additive, which is similar to those in ordered Ba(Zn1/3Ta2/3)O-3 and Ba(Mg1/3Ta2/3)O-3 ceramics. Ordered domains with a twin crystallographic relationship and high-density domain interfaces induced by ordering were observed in the ordered Ba(Cd1/3Ta2/3)O-3 without a boron additive sintered at a relatively high temperature. Atomic resolution TEM further revealed the conservative twin boundaries along (001) and (110) planes and non-conservative antiphase boundaries with a projected displacement vector of the type vertical bar 001 vertical bar in the ordered Ba(Cd1/3Ta2/3)O-3 without a boron additive. Finally, the energetics of different domain interfaces are discussed with the interfacial structures in ordered Ba(Cd1/3Ta2/3)O-3 ceramics revealed by an electron microscope.