Garnet-type lithium-ion conductor Li₇La₃Zr₂O₁₂ (LLZ) has been studied as an electrolyte for all-solid-state lithium batteries because the LLZ exhibits ≥ 3 × 10^-4 Scm^-1 of conductivity at room temperature with chemical and electrochemical compatibility with lithium metal. Three phases have been reported for LLZ; a fast lithium-conduction cubic phase prepared at high temperature (H.T.)-cubic phase stabilized by Al³⁺ substitution, a tetragonal phase, and a low temperature (L.T.)-cubic phase formed by reaction with moisture and CO₂. Although the H.T.-cubic phase exhibits high ionic conductivity, the other phases have low ionic conductivity of ca. 10^-6 Scm^-1 at 25°C. The phase formation of LLZ was investigated by high temperature X-ray diffraction measurements. The thermodynamically stable phase of the stoichiometric LLZ at room temperature is the tetragonal phase. The reversible phase transition between the tetragonal phase and the cubic phase was observed at 640°C for the stoichiometric LLZ. The phase transition temperature decreased with increasing x, and the cubic phase was stabilized at x = 0.25 from room temperature to 900 °C in Li_7-3xAl_xLa₃Zr₂O₁₂. The formation of the L.T.-cubic phase at around 200°C is strongly affected by the Li⁺ /H⁺ exchange reaction with moisture.