The site substitution behavior of the 4d transition elements and the alloying effect of Nb and Mo have been studied using the first-principles discrete variational (DV) and DMol method within the framework of the density-functional theory. The transfer energy calculation indicates that Y, Zr, Nb, and Mo show the tendency to occupy Ti sites while Tc, Ru, Rh, and Pb have the tendency to occupy Al sites in γ-TiAl. In addition, the alloying effect of Nb and Mo on the electronic structure of γ-TiAl is also studied. It was found that Nb and Mo give rise to strong interaction and charge transfer with the neighboring host atoms of TiAl, resulting in a strong solution strengthening effect.