The garnet-type LLZO is one of the most attractive, highly conductive ceramic materials for all-solid-state Li batteries. However, there are so many challenges, such as high temperature sintering for many hours, and air and moisture sensitivity of the materials. In this study, to overcome these obstacles for practical application, we synthesized high conductivity LLZO raw material with Mg, Sr substitution and applied a low temperature sintering process, namely cold sintering, to densify the LLZO-SM-based composites. The LLZO-SM-base composite electrolyte was densified using cold sintering process with DMF as a partially transient liquid phase at 120 °C, and the resulting composite shows a high conductivity of 10−4 S/cm. Moreover, the cold sintered composite electrolyte exhibits high conductivity under a wide range of temperature, as well as good microstructures and grain boundary interfaces. Furthermore, the all-solid-state Li-ion battery with cold sintered composite delivering high capacity was fabricated as a basic demonstration. These promising results demonstrated the feasibility of the cold sintering process for development of solid-state electrolytes and fabrication of all-solid-state Li batteries.