A first-principles study of the electronic structure and magnetic properties of the Heusler compound Mn 2NbAl has been performed. The calculations have been performed by using the self-consistent full-potential linearized augmented plane wave (FPLAPW) method. It is found that the AlCu Mn-2-type structure is preferable energy wise than the CuHg Ti-2-type structure and exhibits half-metallic ferrimagnetism. The calculated total magnetic moment of Mn 2NbAl is 2 mu (B) at the equilibrium lattice constant of 6.005 for the AlCu Mn-2-type structure. The total spin magnetic moment obeys the Slater-Pauling rule and is based on mainly the Mn atoms. The spin-down electrons are metallic, but the spin-up bands are semiconductor with a gap of 0.37 eV, and the spin-flip gap is of 0.09 eV. The AlCu Mn-2-type Mn 2NbAl full-Heusler compound keeps 100 % of spin polarization for lattice constants ranging between 5.93 and 6.20 . The Curie temperature is estimated to be 994 K in the mean field approximation (MFA).