Phase relationships in the BaO-Ga2O3-Ta2O5 ternary system at 1200 degrees C were determined. The A(6)B(10)O(30) tetragonal tungsten bronze (TTB) related solution in the BaO-Ta2O5 subsystem dissolved up to similar to 11 mol % Ga2O3, forming a ternary trapezoid-shaped TTB-related solid solution region defined by the BaTa2O6, Ba1.1Ta5O13.6, Ba1.58Ga0.92Ta4.08O13.16, and Ba6GaTa9O30 compositions in the BaO-Ga2O3-Ta2O5 system. Two ternary phases Ba6Ga21TaO40 and eight-layer twinned hexagonal perovskite solid solution Ba8Ga4-xTa4+0.6xO24 were confirmed in the BaO-Ga2O3-Ta2O5 system. Ba6Ga21TaO40 crystallized in a monoclinic cell of a = 15.9130(2) angstrom, b = 11.7309(1) angstrom, c = 5.13593(6) angstrom, beta = 107.7893(9)degrees, and Z = 1 in space group C2/m. The structure of Ba6Ga21TaO40 was solved by the charge flipping method, and it represents a three-dimensional (3D) mixed GaO4 tetrahedral and GaO6/TaO6 octahedral framework, forming mixed 1D 5/6-fold tunnels that accommodate the Ba cations along the c axis. The electrical property of Ba6Ga21TaO40 was characterized by using ac impedance spectroscopy.