TiAl－based alloys have been regarded as potential structural materials for the aerospace industry because of their high specific strength at elevated temperature, good creep properties and excellent resistance to oxidation. However, their low room temperature ductility and bad high temperature deformability are the considerable barriers to wide application of such alloys. It has been reported that thermo hydrogen treatment (THT) was an effective way to improve the formability of Ti－Al binary alloys. However, up to now few works have been done about the effect of THT on the mechanical properties of utilizable TiAl－based intermetallic compounds. So, the effect of hydrogen on high temperature deformation behaviors of a utilizable TiAl－based intermetallic compound, Ti－45Al－5Nb－0.8Mo－0.3Y alloy, was investigated. Isothermal hot compression tests of the cast Ti－45Al－5Nb－0.8Mo－0.3Y alloy containing 0 and 0.037%H (mass fraction) were carried out by use of a hot simulator at 1150 and 1200℃, with strain rates of 0.1 and 0.01 s^-1, and their microstructures were studied using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results showed that hydrogen was able to decrease high temperature deformation flow stress and refine microstructure, which led to an improvement of the hot formability. Comparing with the unhydrogenated alloy, the hydrogenated alloy presented a reduction of peak flow stress, approximately 25% when deformed at 1200℃ and strain rate of 0.01 s^-1.The lamellar spacing of Hydrogenated Ti－45Al－5Nb－0.8Mo－0.3Y alloy was 265 nm, 77 nm less than that of the unhydrogenated alloy, which was mainly owing to hydrogen－increased β phase content. Besides, the increase of β phase with good high temperature plasticity was one of the main reasons for the decrease of flow stress of the hydrogenated alloy. Hydrogen made the peak strain of true stress and strain curves of the hydrogenated alloy occur earlier than that of the unhydrogenated alloy, which indicated that hydrogen promoted dynamic recrystallization of the hydrogenated alloy to decrease the flow stress. The dynamic recrystallization of β phase was found in the hydrogenated alloy, but not in the unhydrogenated alloy. Comparing with the unhydrogenated alloy, the density of dislocation of the hydrogenated alloy was lower because hydrogen induced the movement of dislocation. More twinning of γ phase occurred in the hydrogenated alloy, which assisted the alloy in high temperature deformation to some extent. In summary, hydrogen－induced dislocation movement, hydrogen－promoted dynamic recrystallization and twinning, and hydrogen－increased β phase content were the main reasons for the decrease of high temperature deformation flow stress of the hydrogenated Ti－45Al－5Nb－0.8Mo－0.3Y alloy.