The effects of low temperature aging on the impact toughness and mechanical properties of Al-1.9%Li-3.0%Cu-0.15%Zr alloy have been investigated. It shows that the impact absorption energy and elongation are increased in the order of over-aged, peak-aged and under-aged specimen at the various aging temperature, and these values in the specimen peak-aged at lower aging temperature are increased. In the specimens aged at 120℃ and 140℃, dimples are observed in the matrix and around the grain boundary. Intergranular fracture has been observed noticeably in the 160℃ peak aged specimen. These results are attributed to the microstructural changes by the low temperature aging treatment that forms more dense and homogeneous distribution of T₁ phases on the dislocations within the matrix and more narrow precipitate free zone in the grain boundary at 120℃ and 140℃ than at 160℃.