When fatigue strength of welded joint is evaluated, it has been assumed that the influence of a structural mean stress on fatigue strength is small as the influence can be neglected due to the existence of welding residual stress. However, the experiences of fatigue damages show that the fatigue strength was affected by the structural mean stress. The influence of structural mean stress on fatigue strength can be evaluated by considering the change in welding residual stress by the shakedown. In this paper, the fatigue assessment method to evaluate the influence of structural mean stress on fatigue strength in design stage is proposed by considering the long-term statistical characteristic of variable stress. It was confirmed that the tendency of occurred fatigue damages could be explained by applying this method. It was also confirmed that the fatigue assessment could be made effectively based on the design S-N curve which has been used widely by applying this method.