In this study, Nb doped gamma-TiAl is designed and examined their tensile properties at elevated temperatures. Small compositional changes cause drastic changes of the mechanical properties at 1273K. In order to clarify the deformation mechanism at elevated temperatures, dislocation structures have been observed using transmission electron microscope (TEM). All of the specimens observed by TEM show that at least two slip systems are operating in each grain. The specimen having relatively lower strength and higher tensile elongations shows more than four operating slip systems in the grains. The specimen having a medium strength shows many super-dislocations and their dissociations that block the other dislocation motions. The specimen having the highest strength shows many deformation twins. These observations suggest that deformation twins become the strong obstacles against moving dislocations at elevated temperatures. The relationships between observed dislocation structures and mechanical properties of gamma-TiAl are discussed.