Abstract: In this study, ultrafine grain/nanocrystalline two-phase γ-TiAl based alloy was prepared by high energy ball milling and vacuum hot pressing sintering. Ti, Al and Nb single powder were mixed together to make the nanocrystalline powder with a nominal composition of Ti-45Al-5Nb (at.%). After ball milling, the nanocrystalline powder was sintered at the temperature of 1200 °C with the pressure of 30 MPa. The sintered microstructure consisted of nanocrystalline α2-Ti3Al and the equiaxed γ-TiAl with grains less than 500 nm. The hot compressive flow behavior of Ti-45Al-5Nb alloy was studied using the Gleeble-1500D thermal simulator at deformation temperatures of 1100°C, 1150°C and 1200°C and strain rates of 1×10-4 s?1, 1×10-3 s?1and 1×10-2 s?1. The results reveal that the peak stresses of equiaxed ultrafine microstructure are significantly lower than the alloys with micron-scale structure. The value of peak stress is reached at the early stage of compression (2.5-3% strain), and the flow stress decreases with increasing temperature and decreasing strain rate. A constitutive equation was established based on experimental data which reflects the structural characteristics of the alloy during hot deformation. It shows that the deformation mechanisms are mainly intracrystalline dislocation in γ-TiAl phase and intergranular twins in γ/γ (001).