La_0.8Sr_0.2Ga_0.8Mg_0.2O_3-δ-based solid electrolytes in which Mg site was partially substituted by Fe, Co or Ni (0.05, 0.1, 0.15 at.％) were fabricated by conventional solid-state reaction and their sintered densities were above 94％ of theoretical density. X-ray diffraction analysis and microstructure observation for the sintered specimens were performed. The ac complex impedance were measured at 400。C to l000。C in air and fitted with a Solatron ZView program. The electrical conductivity of La_0.8Sr_0.2Ga_0.8Mg_0.2O_3-δ-based solid electrolytes substituted by Fe, Co or Ni was higher than that of pure La_0.8Sr_0.2Ga_0.8Mg_0.2O_3-δ. The electrical conductivity of La_0.8Sr_0.2Ga_0.8Mg_0.05Ni_0.15O_3-δ electrolyte was 3.4×10^-2 Scm^-1 at 800。C and the highest value of the whole electrolytes.