AISI 304 austenitic stainless steel specimens were irradiated with high current pulsed electron beam (HCPEB) for 1

5 or 10 pulses. The defect cluster microstructures were investigated by transmission electron microscopy (TEM). While the 1-pulse HCPEB irradiated specimen dose not show any microstructure change

the original microstructure changed completely after the repeated HCPEB irradiation. Dense small defect clusters in sizes of a few nanometers were introduced in the sample

and part of them can be resolve as stacking fault tetrahedral (SFT). For 10-pulse HCPEB irradiation

large-size SFTs normally located at the tip of twin lamellae

and sometimes located in or inside twin lamellae were formed. The largest SFTs were 250 nm approximately. The formation mechanism of large-size SFTs is different from that of the small counterpart. There is a close relationship between large-size SFTs and twin lamellae.