The effect of BaSnO3 on the phase formation and the dielectric properties of Ba2Ti9O20 was investigated as a function of the amount of BaSnO3 in the temperature range of 20 °C to 80 °C at 7 GHz. In the reaction of 2BaCO3 + 9TiO2 → Ba2Ti9O20 + 2CO2↑, the batch with the addition of BaSnO3 enhanced the reactivity compared to the batch with the addition of SnO2. The enhancement of reactivity caused single phase Ba2Ti9O20 to form effectively with less amounts of BaSnO3. As the amount of BaSnO3 increased up to 0.03 mol, the unloaded Q increased due to an increase of the Ba2Ti9O20 phase; for further addition of BaSnO3 over 0.3 mol, the unloaded Q decreased due to the increase of rutile and BaTi4O9 phases. The dielectric constant increased with increasing BaSnO3. As single phase Ba2Ti9O20 was present in the specimen, the Q·ƒ0 value, the dielectric constant, and the TCF were approximately 37,900, 38.7, and 1.7 ppm/°C, respectively.