In impure helium environments, Hastelloy-X is susceptible to carburization and oxidation. These effects are investigated separately, and are related to the creep behavior of the alloy. Experiments were carried out at 900 °C in both helium and air. Carburization resulted in a slight increase of the creep strength up to the onset of the tertial creep. Suppression of the creep crack growth by oxidation was confirmed using notched plate specimens of Inconel alloy 600 and Hastelloy-X. Although the difference of creep strength in air and in helium was very small and considered to be inclusive in the usual scatter, a pessimistic ratio of rupture stress in helium to that in air was estimated to be 0.9.