To provide a fastener made of martensite stainless steel which can present an adequately improved fatigue strength so as to be suitably used for a gas compressor and a gas turbine which are operated at a relatively high working temperature, a plug of a turbine rotor and a method for improving the fatigue strength of the fastener made of martensite stainless steel.

The present method includes a selective heat treatment for substantially improving the fatigue strength without imparing other characteristics required for a fastener. By the heat treatment, a face hardened region 20 preferentially appears on the thread which is nearest to a displacement region 14a provided between a thread 16 and a shank 12 of a fastener 10. In contrast to this, the shank and thread which are remotest from the displacement region 14a is not required to be face hardened. Owing to the face hardening pattern on the thread, it is possible to provide a fastener made of martensite, steel which presents substantially improved fatigue strength compared with a usual fastener.