The microwave dielectric properties and microstructures of Ba₂Ti₉O₂₀-based ceramics were investigated as a function of B₂O₃ content and sintered at 940°C for 2 h. Densities of the specimens were enhanced with an increase of B₂O₃ up to 3 mass% and then decreased. The specimens could be sintered at 940°C without the degradation of the microwave dielectric properties of the specimens. The results of X-ray diffraction (XRD) indicate the presence of five crystalline phases, Ba₂Ti ₉O₂₀, BaZr(BO₃)₂, BaZrO₃, BaTi₄O₉, and Zn₃B₂O₆, in the sintered ceramics, depending upon the amount of B₂O₃ addition. Moreover, the microstructure of Ba₂Ti₉O₂₀-based ceramic with 3 mass% B₂O₃ addition was analysed by using a transmission electron micrograph, which the composition of dendrite phase should be BaZr(BO₃)₂. The specimens with 3 mass% B₂O₃ sintered at 940°C for 2 h showed ε_r of 28.3, Q × f value of 10784 (Q = 1477), and TCF (τ_f ) of -8.2 ppm/°C.