We report the structure and electric properties of La₃Ta_0.5Ga_5.5−xAl_xO₁₄ (LTGAx, x = 0.0 (LTG), 0.3 and 0.5) single crystals, with a Ca₃Ga₂Ge₄O₁₄-type structure, suitable for high temperature piezoelectric application. A single-crystal X-ray structure analysis reveals that Al atoms are distributed in all cation sites except for the decahedral site occupied by La, which favors instead the octahedral site. The piezoelectric moduli d₁₁ kept a constant, whereas |d₁₄| increased 14% with the increasing Al content, x, up to 0.5. The LTGA0.5 crystals showed a low temperature dependence (2% deviation from room temperature to 400°C) of d₁₁ and a higher electric resistivity ρ than those of the LTG crystals. The Al substitution is effective for improving the electrical properties of the LTG.