Al-doped Li7La3Zr2O12 is a promising solid electrolyte material for all-solid-state batteries. In this study, by applying Coulomb energy analysis, density functional theory (DFT) calculations, and thermodynamics considerations, we have investigated defect chemistry in Li6.25Al0.25La3Zr2O12 (Al-LLZO). Defect formation energy plots indicate that decreasing or increasing of Li+ concentration in Al-LLZO is unfavorable. To preserve the charge neutrality, any removed (added) Li+ must be compensated by adding (removing) Li+, which can be viewed as a rearrangement of Li+ ions. The energy cost for Li+ displacement is very low (similar to 0.1 eV), which is in line with the (Li) ionic-conductive nature of Al-LLZO. For a wide range of Li chemical potentials, under Zr poor condition, a complex defect type consisting of 2 added Li+ together with one removed O-2(-) and one removed Zr4+ is the most favorable. Under O poor condition, a Schottky-like defect comprising of a cluster of 2Li(+) and O2- vacancies with approximate to 0.2 eV higher formation energy is the second most favorable defect. In addition, we found that for a narrow range of low (high) Li chemical potentials, removed (added) neutral Li is the most favorable defect type. Our results show that decreasing or increasing of Li content in bulk Al-LLZO is only possible through the formation of complex defects or neutral removed/added Li under Li poor/rich conditions.