For the solid-state reaction synthesis of Al containing Li7La3Zr2O12, various precursors have been used. Since there is a lack of general agreement for choosing precursors, a quantitative approach to build a consensus is required. In this study, a thermodynamic point of view for selecting the precursors in the field of Li7La3Zr2O12 synthesis was covered according to the Gibbs free energy and enthalpy change of precursors' decomposition reactions. In terms of Gibbs free energy change calculations, LiOH, La(OH)(3), and Al(OH)(3) were favorable whereas, LiOH, La2O3, and Al(OH)(3) were the preferred precursors for the enthalpy change calculations. Pellets prepared by using the favored precursors calculated from enthalpy change showed improved densification, higher ionic conductivity (2.11 x 10(-4) S/cm), and lower activation energy (0.23 eV) compared with Gibbs free energy change. As a thermodynamically favored aluminum precursor, Al(OH)(3) was discussed in the present study and hinders the ionic conductivity in comparison to Al2O3.