This paper Investigated the characteristics of residual stress in weld is composed of typical specimens, are investigated by using three dimensional thermal elasto-plastic FEM analysis. Numerically calculated residual stresses in the gas welds were then compared with experimental results obtained by the hole-drilling method. Using the stress amplitude (${sigma}a$)R at the hot spot point of gas weld, the relations obtained as the fatigue test results for typical specimens having various dimensions and shapes were systematically rearranged to obtain the (${sigma}a$)R-Nf relationship. It was found that more systematic and accurate evaluation of the fatigue strength of plug- and ring-type gas-welded joints can be achieved by using (${sigma}a$)R.