The phase formation of Li7 - 3xAlxLa3Zr2O12 was investigated by high temperature X-ray diffraction measurements. The tetragonal phase was obtained in the range of x = 0 to 0.20 and the cubic phase was formed at x = 0.25 at room temperature. A linear change of the lattice parameters was observed from x = 0 to 0.20, which is in good agreement with the systematic composition change. The solubility limit of Al3+ was determined to be x = 0.25. The reversible phase transition between the tetragonal phase and the cubic phase was observed at 640 degrees C for x = 0. The marked increase of ionic conductivity at the phase transition temperature is consistent with the structural change of the ordering/disordering of lithium ions. In addition, the phase transition temperature decreased with increasing x, and the cubic phase was stabilized at x = 0.25 from room temperature to 900 degrees C. The ionic conductivity increased with x and the highest ionic conductivity of 3.1 x 10(-4) S cm(-1) at 25 degrees C was obtained for x = 0.25. Al3+ substitution introduces lithium vacancies, which has a strong influence on the phase transition between the tetragonal and cubic phases. The results obtained here suggest that control of the lithium vacancies is the key to obtain a fast lithium-conducting phase. (C) 2015 Elsevier B.V. All rights reserved.