A-site cation substitution with Ba 2+ and Ca 2+ was made for the complex perovskite Sr(Ni 0.5W 0.5)O 3 to correlate with the structure evolution and the temperature coefficient of dielectric constant. The crystal structures of the solid solutions at elevated temperatures were studied using high temperature X-ray powder diffractometry. A single phase of the perovskite was obtained and ordering of the cations in B-site was observed for all compositions of the solid solutions sintered at 1400 °C for 24 h. The crystal symmetry of the solid solutions at room temperature was changed from monoclinic→rhombohedral→orthorhombic→cubic with the corresponding A-site cation species of Ca 2+→Sr 2+→Ba 2+. The orthorhombic phase of Sr(Ni 0.5W 0.5)O 3 was transformed to the cubic phase at 300 °C. The phase transformation from rhombohedral to cubic at elevated temperatures was observed for each of the Ca 2+-substituted solid solutions, and the phase transformation temperatures increased with increasing Ca 2+ content. Another phase transformation from monoclinic to rhombohedral gradually occurred for (Sr 0.2Ca 0.8)(Ni 0.5W 0.5)O 3 between 400 and 600 °C. Phase transformation was not observed for Ca(Ni 0.5W 0.5)O 3 in the temperature range from room temperature to 1200 °C. Dielectric constants of the solid solutions at room temperature were approximately 27–33. Temperature dependence of dielectric constant was measured in the temperature range from −100 to 500 °C and the temperature coefficients of dielectric constant for each of the solid solutions are discussed correlating with the changes in the crystal symmetry during heating.