Tantalum (Ta) doped Li7La3Zr2O12 (Ta-LLZO) ceramics were prepared via conventional solid-state reaction. LiOH was used as a lithium source and a sintering additive for ceramics. The effect of LiOH on the microstructure and ionic conductivity of Ta-LLZO ceramics was investigated. The results show that LiOH as a lithium source can promote the formation of cubic Ta-LLZO, and LiOH as a sintering additive can effectively improve the densification of ceramics. The dense cubic garnet Ta-LLZO ceramics were obtained by sintering at 1 200 ℃ for 5 h. When the amount of the sintering additive is 6% (in mass fraction), the ionic conductivity of ceramics reaches 6.23×10-4 S·cm-1. It is indicated that Li7La3Zr2O12 prepared via solid-state reaction has a great potential in the application of all-solid-state lithium ion batteries.