There are two stages in fatigue behavior. Those are initiation and growth of fatigue cracks. For crack growth theoretical models exist, but for crack initiation suitable models are limited. The behavior of crack growth can be explained with the Paris' law. After the shape of fatigue crack decided, stress intensity factors can be calculated by means of FEM and so on. Influence of welding residual stress can be handled with the relation of stress ratio and effective stress intensity factor. However, there is no effective theory for the influence of welding residual stress on fatigue crack initiation. Furthermore, the definition of fatigue crack initiation has not been established, because of the difficulty of detecting cracks. The purpose of this paper is to present a simple model for evaluating fatigue crack initiation life. In the model, the stress concentration, welding residual stress and the effect of loading history are taken into account.