A phase relationship of the BaO–Bi₂O₃–TiO₂ system at 1273 K was studied using a solid state reaction. A new compound of BaBi₆Ti₂₀O₅₀ was found in this ternary system. In the BaTiO₃–Bi₄Ti₃O₁₂ pseudo-binary system, the solid solution of Bi₄Ti₃O₁₂ into BaTiO₃ was observed up to 4 mol%  Bi₄Ti₃O₁₂. The lattice parameters of tetragonal BaTiO₃ varied depending on Bi₄Ti₃O₁₂ content in the solid solution range. The structure of BaTiO₃ became cubic with the addition of more than 4 mol%  Bi₄Ti₃O₁₂. The temperature dependence of permittivity obeyed the Curie-Weiss-like behavior in the solid solution range. The highest permittivity of BaTiO₃ decreased and the peak of permittivity became broader with increasing Bi₄Ti₃O₁₂ content.