Ab-initio calculations using the Vienna ab-initio simulation package (VASP) were performed for a high Nb bearing γ-TiAl based alloy with a composition of Ti-47at.%Al-9at.%Nb in order to evaluate the effect of Nb on the crystal structure. The calculations revealed that the c/a ratio of the tetragonal γ-TiAl cell is reduced to ∼1, compared to the binary phase, if Nb atoms occupy Ti- and Al sites. In contrast, the c/a ratio is increased, in comparison to the pure γ-TiAl, if the Nb atoms occupy solely Ti sites and if Ti antisite defects (i.e. Ti atoms on the Al sublattice) are formed. The relaxed structure models were used to perform high-resolution transmission electron microscopy (HRTEM) and high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) image simulations. The results showed that the positions of the Nb atoms should be detectable by these high spatial resolution methods, although it might be easier by HAADF-STEM investigations due to the stronger dependence of the signal on the atomic number Z.