Structure and temperature stability of Ni-Al alloys obtained by mechanical alloying (MA) were studied using X-ray diffraction and differential scanning calorimetry (DSC). MA of Ni-Al powders produces the B2 phase for the composition range 40-61 at.% Ni, and a nanocrystalline supersaturated solid solution Ni(Al) in the range 65-85 at.% Ni. MA of Ni 6 2 . 5 Al 3 7 . 5 leads to a nanocrystalline phase, probably with L1 0 structure. The NiAl (B2) phase is formed through direct rapid exothermic solid state reaction without formation of intermediate solid solutions. Formation of the ordered tetragonal Ni 5 Al 3 phase occurs after prolonged deformation of the nonstoichiometric B2 phase with 62.5 at.% Ni. The results of MA were compared with calculated Gibbs energies of principal phases namely FCC, L1 2 and B2 using the results of computer assessment of the system. The final product of MA is always single phase which has minimal Gibbs energy of the competing phases at alloy composition. This indicates a leading role of the thermodynamic factors on phase formation by MA.