Creep strength of welded joint for high Cr ferritic heat resisting steels decreases due to Type-IV failure in heat-affected zone (HAZ) during long-term use at high temperatures. In order to review the allowable creep strength of these steels, creep rupture data of base metals and welded joints have been collected, and long-term creep strength has been evaluated in the SHC (strength of high-chromium steel) committee in Japan. In the present paper, the creep rupture data of 370 points for welded joint specimens of modified 9Cr-1Mo steel (ASME Grade 91 steel) offered from seven Japanese companies and institutes were analyzed. These data clearly indicated that the creep strength of welded joints was lower than that of base metal due to Type-IV failure in HAZ at high temperatures. From the activities of this committee, it was concluded that the weld strength reduction factor (WSRF) should be taken into consideration for the design and residual life assessment of boiler components in fossil power plants. The committee recommended the WSRF for 100,000 h creep of Gr.91 steel as 0.85 at 575 °C, 0.75 at 600 °C, 0.74 at 625 °C, and 0.70 at 650 °C. The master curve for residual life assessment of Gr.91 steel welds using Larson-Miller parameter was also proposed.