The solid solution's formation and microwave dielectric properties of Bi-doped Ba_6-3xR_8+2xTi₁₈O₅₄ (R: rare earth, x=2/3) series were investigated based on the crystal structure. The crystal structure is the tungstenbronze-type like with perovskite blocks including R ions and pentagonal sites with Ba ions. Bi ions have been substituted for Ba ions in pentagonal sites at early stage in La and Nd-series. Subsequent substitutions of Bi ions occur for R ions in rhombic sites same as other Sm, Eu and Gd series. The Bi ion's substitution affects the microwave dielectric constants (ε_r) as having inflection point for La and Nd-series. In the case of other Sm, Eu and Gd, dielectric constant (ε_r) increased and the quality factor (Q⋅f) value decreased, according to the increase of y in the Ba₄(R_1-yBi_y) _9+1/3Ti₁₈O₅₄ compositions. The temperature coefficients of the resonant frequency (τ_f) has an inflection point at y=0.1-0.15 composition.