Atomic diffusion by the vacancy defect of L1sub.2-Alsub.3M (M = Sc, Zr, Er, Y) was investigated based on a first-principles calculation. The point defect formation energies were firstly evaluated. Then, the migration energy for different diffusion paths was obtained by the climbing-image nudged elastic band (CI-NEB) method. The results showed that Al atomic and M atomic diffusions through nearest-neighbor jump (NNJ) mediated by Al vacancy (Vsub.Al) were, respectively, the preferred diffusion paths in Alsub.3M phases under both Al-rich and M-rich conditions. The other mechanisms, such as six-jump cycle (6JC) and next-nearest-neighbor jump (NNNJ), were energetically inhibited. The order of activation barriers for NNJ(Al-Vsub.Al) was Alsub.3Zr < Alsub.3Y < Alsub.3Er < Alsub.3Sc. The Alsub.3Sc phase had high stability with a high self-diffusion activation barrier, while the Alsub.3Zr and Alsub.3Y phases were relatively unstable with a low self-diffusion activation energy. Moreover, the atomic-diffusion behavior between the core and shell layers of L1sub.2-Alsub.3M was also further investigated. Zr atoms were prone to diffusion into the Alsub.3Y core layer, resulting in no stable core-shelled Alsub.3(Y,Zr), which well agreed with experimental observation.