The effect of hydrogen on phase transformation and mechanical properties of the unhydrogenated Ti-45Al-5Nb-0.8Mo-0.3Y (in at.%) alloy and Ti-45Al-5Nb-0.8Mo-0.3Y alloy with 1.5 at.% hydrogen addition is investigated in the temperature range 1323-1473 K with a strain rate of 0.01 s(-1). The results show that the flow stresses of the hydrogenated alloy are lower than those of the unhydrogenated alloy. H as an interstitial atom occupies B2 phase tetrahedral interstice. The clearance radius of tetrahedron of B2 phase is decreased by approximately 4.6% after B2 -> omega(o) transformation. So, H can impede such phase transformation by means of impeding the decrease of clearance radius of tetrahedron. The volume fractions of B2 phase of the hydrogenated alloy are more than those of the unhydrogenated alloy under all deformation conditions due to hydrogen-stabilized beta phase. The lower flow stress of the hydrogenated alloy compared with the unhydrogenated alloy is mainly attributed to hydrogen-increased B2 phase, hydrogen-induced twinning and stacking faults of gamma phase, and hydrogen-impeded formation of omega(o) and alpha(2) phases. Copyright (C) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.