The piezoelectric properties of a solid solution, x(Bi1/2Na1/2)TiO3-(1-x)(Bi1/2K1/2) TiO3 + La2O3 y wt% [BNKTx + La2O3 y wt%; x = 0.50, 0.70, y = 0–0.6] were investigated, focusing on piezoelectric constant, d 33, and depolarization temperature, T d. The dielectric constant increased with increasing La2O3 content (y). The highest electromechanical coupling factor, k 33 (0.493) and the highest d 33 (153 pC/N) was obtained in BNKT0.70 + La2O3 0.2wt%. The T d of BNKT0.70 + La2O3 y wt% was almost the same with y = 0–0.3 at 210–220°C and decreased with y ≥ q 0.4. BNKT0.70 + La2O3 0.2 wt% (d 33 = 153 pC/N and T d = 219°C) is a good candidate for practical use in actuators.