A series of complex perovskite solid solutions of Ba[(Mg_1−xCd_x)_0.33Nb_0.67]O₃ have been synthesized by the columbite method. Detailed Rietveld refinement of their X-ray diffraction data show that Ba[(Mg_1−xCd_x)_0.33Nb_0.67]O₃ has an order trigonal structure. The ordering degree as determined by the B-site occupancies increases with the partial substitution of Cd for Mg. However, a decrease in the ordering degree in the Ba(Cd_0.33Nb_0.67)O₃ sample is observed, which can be attributed to a relatively lower synthesis temperature. All the impurity phases are successfully identified by X-ray quantitative phase analysis. Dielectrics properties at low frequencies for all the Ba[(Mg_1−xCd_x)_0.33Nb_0.67]O₃ compounds have been measured successfully.