The effects of irradiation on the fatigue properties of stainless steel were studied under the condition of in situ irradiation or after irradiation using 20 MeV protons. Strain controlled fatigue tests using a resonant and a drive force DF at constant were measured in situ during the test. Whereas the present damage rate was very low, the apparent effects of in situ irradiation on the fatigue properties were observed, e.g. the elongation