Li7La3Zr2O12 (LLZO) has been reported to react in humid air to form Li2CO3 on the surface, which decreases ionic conductivity. To study the reaction mechanism, 0.5-mol Ta-doped LLZO (0.5Ta-LLZO) pellets are exposed in dry (humidity similar to 5%) and humid air (humidity similar to 80%) for 6weeks, respectively. After exposure in humid air, the formation of Li2CO3 on the pellet surface is confirmed experimentally and the room-temperature ionic conductivity is found to drop from 6.45x10(-4)Scm(-1) to 3.61x10(-4)Scm(-1). Whereas for the 0.5Ta-LLZO samples exposed in dry air, the amount of formed Li2CO3 is much less and the ionic conductivity barely decreases. To further clarify the reaction mechanism of 0.5Ta-LLZO pellets with moisture, we decouple the reactions between 0.5Ta-LLZO with water and CO2 by immersing 0.5Ta-LLZO pellets in deionized water for 1week and then exposing them to ambient air for another week. After immersion in deionized water, Li+/H+ exchange occurs and LiOH H2O forms on the surface, which is a necessary intermediate step for the Li2CO3 formation. Based on these observations, a reaction model is proposed and discussed.