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First principles calculations have been performed to study the mono-vacancy formation energies and electronic properties of M2AlC (M = V and Nb) compound. The results show that the M mono-vacancy has a maximum formation energy. While the C mono-vacancy has a minimum formation energy, which means that the C mono-vacancy is the energetically most favorable in M2AlC. The d-electrons of M element contribute most to the DOS of M2AlC around the Fermi level, it implies that the conductivity of M2AlC comes from the transition metal M. The M-C bond is stronger than the M-Al bond, which is caused by the strong hybridization energy peak between M and C atom. In addition, the M-C bond is weaken in the presence of the M or C mono-vacancy. The cell volumes are reduced when the mono-vacancy is formed. These results help us to understand the origin of the defect-related properties and phase stability of V2AlC and Nb2AlC under extreme environment.