AbstractStructural stainless steels for cryogenic use in superconducting magnets for fusion reactors are used under high magnetic field and stress. The authors have performed fracture thoughness tests, using the unloading compliance method at 4.2 K on SUS304, 316LN and 304L stainless steels precracked at 77 K, in a varying magnetic field (0 and 8 T). Fracture toughness values (JIC) for 304 and 316 LN steels at 8 T decreased by 17 and 20%, respectively, as compared with the 0 T condition. On the other hand, JIC for 304L steel was 30% increased by applying a high magnetic field of 8 T. Details of martensite formation and austenite stability are considered in discussion.