Microstructural studies of the complex perovskite compound La(Mg2/3Nb1/3)O-3 (LMN) were conducted using transmission electron microscopy (TEM) and X-ray diffractometry (XRD) at elevated temperatures, 1:1 chemical ordering of B-site cations and tilting of oxygen octahedra were observed in LMN, Three types of superlattice reflections, [1-2]{111}, [1-2]{110}, and [1-2]{100} were observed at room temperature and at 800 degrees C in electron diffraction patterns. In the XRD experiments, the [1-2]{210} and [1-2]{300} extra peaks disappeared at temperatures > 1200 degrees C. However, the intensity of the superlattice [1-2]{111} peak did not change with increased temperature up to 1400 degrees C. These results strongly indicated that the origin of superlattice reflection [1-2]{111} was different from that of the other superlattice reflections. It was mainly caused by the 1:1 chemical ordering of magnesium and niobium atoms. The TEM image observed at 800 degrees C showed the ordered domain structures separated by the antiphase boundaries.