The crystal structure of La(Mg2/3Nb1/3)O-3 has been analyzed by high-resolution transmission electron microscopy (HRTEM) and Rietveld refinement using X-ray and neutron powder diffraction patterns. According to our analysis, La(Mg2/3Nb1/3)O-3 has the 1:1 ordered monoclinic P2(1)/n structure, contrary to the results of previous works which suggest a 1:1 ordered rhombohedral R (3) over bar or orthorhombic Pbnm structure. The lattice parameters of La(Mg2/3Nb1/3)O-3 are a = 5.6052, b = 5.6482, c = 7.9414 Angstrom, and beta =- 89.9501 Angstrom, Monoclinic La(Mg2/3Nb1/3)O-3 is distorted by the inphase and the antiphase tilting of octahedra with the a(-)a(-)c(+) tilting system. The antiparallel shift of A-site cations was also found in La(Mg2/3Nb1/3)O-3