Nonmagnetic austenitic stainless steel is the candidate alloy for structural material in superconductive magnets which can control the plasma in a fusion reactor because this steel has widely used cryogenic applications. Therefore, strength and toughness are required to guarantee the structural integrity during operation. However, it is often hard to evaluate correct fracture toughness at 4K since serration occurs due to martensitic transformation. This phenomenon can cause not only a temperature rise at the crack tip but also extra deformation. This paper presents the effect of the strain rate on fracture toughness at 4K relating to martensitic transformation. It is found that the fracture toughness decreases with the increase of strain rate although the material ductility is apparently promoted by martensitic transformation. The difference in fracture mechanisms corresponding to each strain rate is also discussed based on SEM examination.