The electron-irradiation studies on Hastelloy-X in the temperature range of 400~600°C to a dose of about 40 dpa have revealed that Hastelloy-X is much more resistant to void swelling compared to Type 316 stainless steel. The low void swelling in Hastelloy-X is attributable to small void size which arises from long incubation period for void nucleation and low climbing rate of dislocations. The magnitude of the incubation period depends on the stability of void embryo; the void in Hastelloy-X is not considered to be stabilized with solute atom segregation as seen in Type 316 stainless steel, and then, the high dislocation density is essential for the void nucleation. The stabilization of void embryo also affects the void number density, especially at high temperature where vacancy supersaturation rate is low. And the small void number density in Hastelloy-X results in very small void swelling at a high temperature (600°C).