Dielectric properties and defect chemistry of (Ba1−xLax)(Ti1−xLux)O3 were studied. Solid solubility (<0.1) is lower than (Ba1−xLax)(Ti1−xRx)O3 (R = Tb, Dy) (x = 0.15). A regularity is discovered that tan δ decreases with Ti-site lanthanide contraction. The x = 0.05 ceramic with Y5S specification exhibits a lowest tan δ (= 0.01). The defect chemistry is provided.