The dielectric and physical properties of the complex perovskite Ba(Mg1/3Ta2/3)O3 system in which magnesium was substituted for nickel from 0.03-0.67 mol%, were investigated in the temperature range 20-110-degrees-C, and the frequency range 10.5-14.5 GHz. As the nickel content was increased, the dielectric constant, the degree of ordering, and the unloaded Q decreased. The temperature dependence of the dielectric constant and the temperature coefficient of resonant frequency of the specimens annealed at 1500-degrees-C for 20 h were found to be greater than those of the specimens sintered at 1650-degrees-C for 2 h. These results are due to the increase in the density, the increase in grain size, and the lattice distortion.