The valence electron structures, theoretical binding energies and forming enthalpy of phases in Ni-Al system were calculated based on the empirical electron theory of solids and molecules, and the interfacial reaction was predicted through comparison of the forming enthalpy of phases. The calculated binding energy values of Ni, Al, Ni3Al, Ni5Al3, NiAl, Ni2Al3 and NiAl3 are 427.44, 324.58, 441.11, 440.10, 435.51, 432.66 and 395.05 kJ/mol, which are in good agreement with experimental values. At the Ni/Al interface, Ni2Al3 phase has a minimum enthalpy of formation, so it will be the first to precipitate in heat process; then, the NiAl phase may be formed at the Ni2Al3/Ni interface, while the Ni3Al phase may be formed at the NiAl/Ni interface. At the NiAl/Ni3Al interface, the absolute value of forming enthalpy of Ni5Al3 is small, so its formation needs high temperature and long time. The predicted results agree well with the experimental.