A study was conducted to examine the effects of In addition on the precipitation behaviors of Al-2.1Li-2.9Cu alloy by differential scanning calorimetry, transmission electron microscopy and microhardness tester. DSC analysis was measured over the temperature range of 25∼50℃ at a heating rate of 2∼20℃/min. The heat evolution peaks due to the formation of GP zone and δ` phase shift to higher temperature and the peaks to T₁and θ` phases shift to lower temperature by In addition. From this result, it was proved that the formation of GP zone and δ` phase is suppresed whereas that of T₁and θ` phases are accelerated by the In addition of 0.15wt%. The age hardening curve aged at 190℃ showed that the In bearing alloy(alloy B) has more faster age hardening response and a higher peak hardness than In-free alloy(alloy A), attributed to the fine and homogeneous distribution of T₁and θ` phases. The activation energies for the formation of δ`phase in In-free and In-bearing alloys are 22.3㎉/㏖ and 18.6㎉/㏖, respectively. Those for T₁(+θ`) phase of In-free and In-bearing alloys are 24.3 and 37.5㎉/㏖, respectively. Quenched-in excess vacancies play an important role to the formation of precipitates.