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Summary: The superiority of garnet‐type solid electrolyte makes it one of most promising candidates for all‐solid‐state lithium batteries. Several studies show that introduction of alumina during synthesis can greatly improve the density and ionic conductivity of garnet electrolyte Li7La3Zr2O12 (LLZO), but the reason of poor sinterability of LLZO is still unclear. In this study, we reveal that lithium carbonate, which has a high decomposition temperature and covers on the particle surface of LLZO, is the underlying reason that handicaps the sinterability of Li‐Garnet electrolyte in air. The addition of alumina promotes the decomposition of Li2CO3 (down to 400°C) and the concomitant product LiAlO2, as a fast Li‐ion conductor, facilitates the sintering process and bulids a fast Li‐ion conducting network along the grain‐boundaries, significantly increasing the ionic conductivity of Li‐Garnet electrolyte. By the conventional solid state sintering, the 10 mol% Al2O3 modified Li6.4La3Zr1.4Ta0.6O12 (LLZT‐10Al2O3) electrolyte reaches a relative density of 96% and shows a conductivity of 0.31 mS cm−1 at room temperature. The prepared LLZT‐10Al2O3 electrolyte exhibits a good wetting property toward metallic Li electrode with an interfacial resistance of 59 Ω cm2 compared to 1270 Ω cm2 for LLZT/Li. This work provides a fundamental understanding and a valuable strategy for developing high performance garnet‐type electrolyte for all‐solid‐state lithium batteries. [ABSTRACT FROM AUTHOR]