Magnetic domain structures and crystal structures of double perovskite Ba(2)FeMoO(6) were studied using Lorentz transmission electron microscopy (TEM), dark-field imaging and high-resolution TEM. Two types of magnetic domain structure were observed below the Curie temperature: an ordinary 180 degrees ferromagnetic domain structure and a typical maze-pattern structure. In regions where maze-pattern structures were observed, antiphase boundaries were found by dark-field imaging and high-resolution TEM and develop densely in Fe/Mo-ordering domains, suggesting that Fe/Mo antisite defects affect magnetic domain structures.