The impact of point defects on the elastic properties of off-stoichiometric L12-Al3Li intermetallic was investigated through the first principle pseudopotential calculations. The results show that at the temperature of 600K, the AlLi and LiAl anti-sites dominate in the equilibrium defect concentrations in Li-rich and Al-rich L12-Al3Li, respectively. In the whole composition range of interest, the concentrations of anti-site defects (both AlLi and LiAl) far outnumber those of vacancies (VLi and VAl). The defective stoichiometric L12-Al3Li is predicted to be intrinsically brittle. The Al-rich L12-Al3Li has the increasing LiAl and VLi densities that tend to reduce the hardness and brittleness, while the Li-rich L12-Al3Li does the reverse.