The effects of the replacement of Sr by Ba for Sr(Ga_1/2Ta_1/2)O₂ ceramics (SGT) on microwave dielectric properties and microstructures were investigated. With the increase of the replacement of Sr to Ba, a single perovskite phase changed to mixed crystalline phases, which were composed of (Ba, Sr)₅Ta₄O₁₅ or unknown phases in addition to a perovskite phase. Microwave Q and temperature coefficients of resonant frequencies (τf) were improved with the replacement of Sr by Ba. (Ba_0.6, Sr_0.4)(Ga_1/2Ta_1/2)O₃ ceramics showed a dielectric constant of 27.7, Q value of 10000 at 9GHz and τf=-4ppm/°C. The relationship between microwave Q and microstructures is discussed.