Cubic-phase garnet-type Li7La3Z2O12 is a promising candidate for an electrolyte of all-solid-state lithium-ion batteries; however, its poor sinterability due to Li sublimation during firing has impeded large scale development. This study demonstrates a solid-state reactive sintering (SSRS) process with added CuO as a sintering aid to enable enhanced materials processing at lower temperatures. Applying the SSRS process with the addition of 1 wt% CuO decreased the sintering temperature for 0.5 mol%Ta-doped LLZTO pellets having over 90% relative density from 1250 to 1100 °C to reduce Li loss. The 1 wt% CuO addition did not lead to secondary phase formation as detected by XRD, nor to appreciable electronic conduction below 100 °C as measured by four-point probe method. The 1 wt% CuO-mixed LLZTO pellet exhibited high conductivity of approximately 3.0 × 10−4 S·cm−1 (bulk) and 5.45x10−5 S·cm−1 (grain boundary). The mechanism of CuO function as a sintering aid is presumed to be enabling liquid-phase sintering along with enhancing the decomposition of LiOH. The combined SSRS process along with optimized CuO sintering aid addition is a one-step process that is a practical technique to enhance the preparation of LLZO-based electrolyte for all-solid-state lithium-ion batteries.