Dynamic creep and fatigue tests were carried out with an 18-8 Mo-Cb steel at a temperature of 650°C. The results were discussed from the standpoint of the analysis, which had been proposed by the authors previously, concerning the prediction of dynamic creep and fatigue strength from the information on static creep and creep rupture tests together with reversed stress fatigue data. The prediction by the analysis showed a satisfactory agreement with the experimental results as for a practical purpose, although a little discrepancy was observed due to the acceleration of the precipitation hardening by alternating stress. In order to contribute to clarification of the relation between the precipitation hardening characteristics of the material and its strength characteristics, the electron-microscopic observation of structure, hardness test and creep rupture test after the dynamic creep were carried out. Their results were discussed in connection with the discrepancy between the analysis and experiments.