Long-term creep rupture life is usually evaluated from short-term data by a time-temperature parameter (TTP) method. The allowable stress of Gr.122 steel listed in the ASME code has been evaluated by this method and is recognized to be overestimated. The objective of the present study is to understand the causes of the overestimation and propose appropriate methodology for avoiding the overestimation. The apparent activation energy for rupture life of the steel changes from a high value of short-term creep to a low value of long-term creep. However, the decrease in is ignored in the conventional TTP analyses, resulting in the overestimation of rupture life. A multiregion analysis of creep rupture data is employed to avoid the overestimation; in the analysis creep rupture data are divided into a couple of regions so that the value is unique in each divided region. The multiregion analysis provides a good fit to the data and the lowest value of creep rupture strength among the three ways of data analysis examined. A half of 0.2% proof stress cannot provide an appropriate boundary for dividing data to be used in the multiregion analysis. In the 2001 edition of the ASME code an average concept has been proposed as a substitution for the safety factor of for average rupture stress. The allowable stress of Gr.122 steel changes significantly depending on the allowable stress criteria as well as the methods of rupture data analysis: i.e., from .