The current ASME Boiler and Pressure Vessel (B&PV) Code Section III, Division 5, Subsection HB, Subpart B has only one design fatigue curve for grade 91 steel (Gr. 91) at 540 °C (or 1000 °F). The ASME Section III Working Group on Creep-Fatigue and Negligible Creep (WG-CFNC) has taken an action to incorporate the temperature-dependent design fatigue curves for Gr. 91 developed by Japan Society of Mechanical Engineers (JSME) into ASME Section III Division 5. During the process, issues regarding the effect of mean stress on fatigue analysis, and how to consider the mean stress effect for elevated-temperature design, were brought up. To evaluate whether the design fatigue curves of Gr. 91 needed adjustment to account for mean stress, critical tests were designed and performed at 371 °C (700 °F) and 540 °C (1000 °F). This study is similar to the work performed on Alloy 617 when its fatigue design curves were established for temperature range of 538–704°C (1000–1300°F) as part of the Code Case package for Alloy 617 to be used as Class A construction material in Division 5. The effects of mean stress on Alloy 617 were evaluated at 550°C (1022°F). The results showed that the mean stresses introduced by the non-zero mean strain could not be maintained under strain-controlled fatigue and resulted in negligible effect on the fatigue life. Mean stress correction was not recommended for Alloy 617 fatigue design curves in Division 5. This study shows the same conclusion for Gr. 91.