The 1:1 structural order and B-site cation order in Pb(Mg_1/3Nb_2/3)O₃ ceramics was significantly enhanced by Zr⁴⁺ substituting Nb⁵⁺ according to the chemical formula Pb(Mg_1/3Zr_2x/3Nb_2(1−x)/3)O_3−x/3. The enhanced structural and cation order was revealed by both X-ray diffraction and transmission electron microscopy. The increased charge difference and the introduced oxygen vacancies were suggested to account for the observed long-range cation order. Typical relaxor ferroelectric behavior was retained even in the ceramics with a high degree of cation order.