Crystal structures of (Ba1-xLax)[Mg(1+x)/3Nb(2-x)/3]O-3 (BLMN) with 0.3 less than or equal to x less than or equal to 0.7 were analyzed using the X-ray Rietveld refinement. The evolution of the X-ray powder pattern could be explained by the presence of 1:1 ordering of the B-site cation and the antiphase tilting of the oxygen octahedra, which were further confirmed by the observation of the antiphase boundary (APB) for the ordering and the ferroelastic domain boundaries for oxygen tilting by TEM. The crystal structure, which has only the 1:1 ordering of the B-site cation, has the space group of Fm (3) over barm (225), and that which has both the 1:1 ordered and the antiphase tilting of oxygen octahedra has the space group of I4/m (87). For the quantitative analysis of X-ray diffraction data, a new concept of the two-phase mixture of phase (Fm (3) over barm) and phase (I4/m) was employed in these systems. The obtained results were better than others, which employed only one single phase. The molar weight of the I4/m phase in a mixture increased as the La content in the BLMN system increased from 11.5% (x = 0.3) to 24.3% (x = 0.7).