The characteristic properties including equilibrium lattice constants, elastic constants, cohesive energies and alloy formation energies, of stoichiometric L12 type NiAl3, L12 type Ni3Al, L10 type NiAl and B2 type NiAl, are calculated using a constructed n-body potential model combined with molecular dynamics. Furthermore, the types of point defects in the ordered NixAl1-x (x=0.25, 0.5, 0.75) systems in the nonstoichiometric case are discussed, to validate the model. The present calculation shows that the point defects are the antisites. It is in good agreement with other investigations. Then the n-body potential calculations are performed to investigate the site preference of ternary elements Mo/Ta/W alloying additions in the NixAl1-x systems. The occupation site of the ternary elements are analyzed based on the occupational energy minimizing. And the results show obviously that the occupational sites of Mo/Ta/W are the Al site.