Li7La3Zr2O12 (LLZ) solid electrolyte is one of the promising electrolytes for all-solid-state battery due to its high Li ion conductivity and stability against Li metal anode. However, high calcination temperature for LLZ preparation promotes formation of La2Zr2O7 impurity phase. In this paper, an effect of Al2O3 addition as sintering additive on LLZ solid electrolyte preparation and electrochemical properties of Al2O3-added LLZ were examined. By the Al2O3 addition, sintered LLZ pellet could be obtained after 1000°C calcination, which is 230°C lower than that without Al2O3 addition. Chemical and electrochemical properties of the Al2O3-added LLZ, such as stability against Li metal and ion conductivity, were comparable with the LLZ without Al2O3 addition, i.e. σ bulk and σ total were 2.4×10−4 and 1.4×10−4 Scm−1 at 30°C, respectively. All-solid-state battery with Li/Al2O3-added LLZ/LiCoO2 configuration was fabricated and its electrochemical properties were tested. In cyclic voltammogram, clear redox peaks were observed, indicating that the all-solid-state battery with Li metal anode was successfully operated. The redox peaks were still observed even after one year storage of the all-solid-state battery in the Ar-filled globe-box. It can be inferred that the Al2O3-added LLZ electrolyte would be a promising candidate for all-solid-state battery because of facile preparation by the Al2O3 addition, relatively high Li ion conductivity, and good stability against Li metal and LiCoO2 cathode.