Microstructural studies on the domain boundaries in Ca(Mg1/3Nb2/3)O-3 (CMN) complex perovskite compound were conducted using X-ray diffractometry and transmission electron microscopy, The 1:2 chemical ordering of B-site cations and the tilting of oxygen octahedra, were involved in the CMN microstructure, as inferred from the presence of two types of domain boundaries. One type was the antiphase boundaries (APBs), which did not lie on a specific see of crystallographic planes. These boundaries were caused by the chemical 1:2 ordering of B-site cations, magnesium and niobium. The other type was the ferroelastic domain boundaries, which were parallel to a certain crystallographic plane. Therefore, CMN had the 1:2 ordered monoclinic unit cell distorted by the antiphase or in-phase tilting of oxygen octahedra. CMN had the mixed phases rather than the homogeneous phase.