The hot deformation behavior of as－cast Ti－43Al－9V－Y alloy has been studied using thermo－simulator system Gleeble－1500D in the temperature range 1100－1225℃ and strain rate range 0.01－1 s^-1. It was found that the flow behavior of Ti－43Al－9V－Y alloy depends on the deformation temperature and strain rate. The flow stress－strain curves exhibit oscillations in the stage of flow softening. According to principles of dynamic materials modeling (DMM), processing maps were constructed on the basis of flow stress data. Wedge cracking was expected to be observed in the region of high strain rates with low peak efficiency (<40%). The α+β+γ→β+γ phase transformation occurring at 1150℃ and 0.01 s^-1 contributes to the peak efficiency of around 60 %. The dynamic recrystallization occurred at 1200－1225℃ and 0.01－0.05 s^-1 with a peak efficiency of around 80%, which is the optimum condition for the hot deformation of Ti－43Al－9V－Y alloy. Combining with the titanium and aluminum phase graphs, it could be found that 1225℃ located at the single－phase region ofα phase. The excellent hot deformability of disorderedαphase is considered to be the main reason for the improvement of hot workability of TiAl alloys. As－forged alloys have good room－temperature ductility and fracture toughness simultaneously. In addition, it has been found that B2 phase could hinder the propagation of crack without decreasing the plasticity at room temperature.