We observe the influence of Al occupancies in Li sites on the formation process of the garnet solid electrolyte of Li7La3Zr2O12 (LLZO). A direct incorporation of Al is first promoted in a Li-insufficient garnet solid electrolyte during the calcination process of 850 °C and then the cubic phase of LLZO is obtained after successive annealing step of 1000 °C. Comparing to pristine LLZO, Al incorporated LLZO shows less formation of Li2CO3, keeping crystallographic and physicochemical properties. This Al incorporation improves both the ionic conductivity and interfacial resistance to poisoning procedure.