The influence of ternary additions Cr, Fe, Mn, Ni, Zr, Nb, Mo, Hf, Ta, Si, Ga, Ge, In, and Sb, as well as the anti-site defects of both Ti and Al, on lattice parameters of TiAl were studied by the first principles electronic structure calculations with a discrete variational cluster method. The results of the calculation show that the effect of ternary additions on the distortion of TiAl lattice varies with the substitution behavior of the individual alloying element involved. The addition of alloying elements in TiAl causes a change in the electronic structure and the density of states of the system and results in variation of the bond strength between the atoms. The total and partial density of states (DOS) of binary TiAl and of ternary TiAl–M, M = Cr, Zr, and Sb, etc., were comparatively examined. The relationship between the DOS and the bond strength is discussed. The present work suggests that the origin of the lattice distortion of the ternary TiAl–M systems lies in the variation of the electronic structure.