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It is important to develop strength evaluation procedures for the fatigue and creep fatigue of 2.25Cr-1Mo weld joints. For this purpose, studies have been conducted on the low cycle fatigue and creep fatigue strength of full penetrated welded joints at elevated temperatures. Specimens and testing conditions were determined to assess the applicability of typical welded structural discontinuity. Fatigue and creep fatigue tests have been carried out at 425 deg C and 500 deg C in order to evaluate the strength of weld joints in elevated temperatures. Further, the relationship with the local strain range, Delta(epsilon) at the toe of the weld where crack initiation occurs, was determined. The local strain range at the toe of the weld was estimated by both measurement and inelastic FEM analysis. As a result, the crack initiation life, Nc is able to estimate by using the local strain range Delta(epsilon) which consider the material discontinuity through the S-N curve for smooth base metal specimens. The methodology to estimate the structural integrity simply is discussed with regard to some estimation method of inelastic strain. It is shown that on the part from which the material characteristic changes like welded joints, there occur strain by material discontinuity and a strain concentration changes in discontinuous. When a model is made only of the base metal, the generating position of the maximum strain is different. When creep fatigue damage is evaluated for welded joint, there is good correlation between the actual strain that influenced by material discontinuity and the maximum strain in case of there is not material discontinuity. It can be evaluated in the examination range of this study as a ratio 1 of amplification. In the design, strain amplification factor 2 for material discontinuity is recommended. (VDEh)