High-purity Fe-Cr-Ni austenitic alloys corresponding to commercial Type-316 stainless steel and Hastelloy-X were used to investigate the void swelling mechanism of the austenitic steels. The alloys were irradiated with 1 MeV electrons in a high voltage electron microscope in the temperature range of 300~600°C to a total dose of about 30 dpa. Low void swelling in Ni-base alloy is attributed to both low void number density and small void size. Void embryos in Fe-base alloy are stabilized by strain field arised from Ni solute segregation around the void surface. The stabilization does not occur in Ni-base alloy, which results in extremely low void number density at high temperatures (>500°C). Higher void growth rates in Fe-base alloy than in Ni-base alloy are attributable to large climbing rate of dislocations produced during the irradiation.