The aim of this work is to probe the helium induced defect production and accumulation in 40 keV He+ irradiated polycrystalline W and its alternative alloy W- 5wt.%Ta using transmission electron microscopy (TEM) combined with in-situ helium irradiation at 800°С. A maximum damage level of 1 dpa with a maximum He-to-dpa ratio of 5.5 at%/dpa has been reached in this work for both materials, which corresponds to an ion fluence of 7.33×1016 He+ /cm2 . The presence of radiationinduced dislocation loops was not observed at this temperature. The low density of the incipient bubbles in W has been already detected at 0.004 dpa, which corresponds to a fluence of 3.3×10 14 He+ /cm2 . The experiments conducted at 800 °C have shown that the addition of 5wt.% of tantalum into tungsten may diminish the binding of He ions with vacancies into complexes, which serve as the core of the bubble, thus hindering helium bubble formation below 0.02 dpa and their further growth and population at higher damage levels. By exceeding the damage dose 0.3 dpa, a progressive transition from a spherical to a faceted shape of the bubbles has been observed in W but not in the W-5Ta alloy. At 1 dpa, >80% of the bubbles in W were of the faceted type with the facet planes of {110}.