Microstructural studies of the domain boundaries in the complex perovskite compound lanthanum magnesium niobate (La[Mg2/3Nb1/3]O-3, LMN) were conducted using transmission electron microscopy. Both the 1:1 chemical ordering of B-site cations and the tilting of oxygen octahedra affected the domain boundaries. Two types of domain boundaries were observed. In addition to the presence of antiphase boundaries, which were insensitive to the crystallographic planes, ferroelastic domain boundaries that were caused by the phase transition due to the tilting of oxygen octahedra also were present. In some grains, only one type of oxygen tilting was present, which resulted in a single domain in one grain. Two or three domains were observed in a grain where the walls were parallel to the {110} plane. Many domains also were observed in a grain that had boundaries whose linear characteristics were gradually reduced.