Using the first-principles pseudopotential plane-wave methods based on the density functional theory,the geometric and electronic structures of(NiX)Al supercells with different alloying elements X(X=Cr,Mn,Fe,Co and Cu) or different Fe contents,e.g.0,3.125%,4.167% and 6.25%(mole fraction) are calculated.Several mechanical properties parameters such as elastic constant C_(44),Cauchy pressure parameter(C_(12)-C_(44)),elastic modulus E,the shear modulus G and their ratio G/B_0 are used to characterize and assess the effect of alloying element and Fe content on the ductility and hardness of NiAl intermetallic compounds,respectively.The result shows that high addition of alloying element X(6.25%) can not enhance the ductility of NiAl intermetallic compound although it makes the hardness of NiAl improve as following order: NiAl<(Ni_7Cu)Al_8<(Ni_7Cr)Al_8<(Ni_7Fe)Al_8<(Ni_7Co)-Al_8<(Ni_7Mn)Al_8.And the decreasing order of ductility of NiAl induced by alloying element X is opposite to their hardness increasing order.With the increase of Fe content,the hardness of NiAl is elevated.And the upper limit making the hardness of NiAl increase is about 4%.Meanwhile a gradual improvement of the ductility of NiAl is also observed as Fe addition content decreases.From this variation tendency of the ductility of NiAl versus Fe content,a deduction is made.The intrinsic embitterment of NiAl can be restrained if Fe content is enough low.