Properties of various defects of He and H atoms in W Ta alloys are investigated based on density functional theory.The tetrahedral interstitial site is the most configured site for self-interstitial He and H in W and W-Ta alloys.Only a single He atom favors a substitutional site in the presence of a nearby vacancy.However,in the coexistence of He and H atoms in the presence of the vacancy,the single H atom favors the tetrahedral interstitial site (TIS) closest to the vacancy,and the He atom takes the vacancy center.The addition of Ta can reduce the formation energy of TIS He or H defects.The substituted Ta affects the charge density distribution in the vicinity of the He atom and decreases the valence electron density of the H atoms.A strong hybridization of the H s states and the nearest W d state s exists in W53He1H1 structure.The sequence of the He p projected DOS at the Fermi energy level is in agreement with the order of the formation energy of the He-H pair in the systems.