Abstract: General primary membrane stress (S_mt ) of material is the important parameter used for the mechanical analysis of high temperature reactor design, and the data of Smt at 300000 hours in the ASME code and RCC-MR code cannot meet the needs of long-life nuclear reactor design. Based on the data of allowable stress intensity (S_t), expected minimum stress-to-rupture (S_r ) and stress rupture factors (R ) at 300000 hours in the ASME code, the Smt at 500000 hours of 316 stainless steel base metal and weld required by long-life reactor design are obtained successfully by Larson-Miller extrapolation model.