Fatigue failure sometimes occurs in welded joints due to stress concentration and tensile residual stress at the weld toe. Therefore, some type of treatment to delay fatigue crack initiation and propagation is necessary in order to enhance the fatigue life of welded steel structures. The hammer peening process is well known as one method for improving the fatigue life of welded joints by generating a compressive residual stress field near the weld toe, which is recognized as the fatigue crack initiation site. When considering fatigue life improvement of large welded structures in which a compressive residual stress field is applied near the weld toe, not only improvement of fatigue crack initiation life but also extension of fatigue crack propagation life must be discussed. To date, however, there have been little research which deals with fatigue crack estimation in a compressive residual stress field. In this study, the mechanism of fatigue crack propagation in a compressive residual stress field was clarified by experimental observation of crack propagation from a surface crack after hammer peening. The morphology of propagating surface cracks was measured in plate and gusset welded joint specimens by the beach mark method. The results clarified the fact that the morphology of a surface crack which propagated in the compressive residual stress field was different from that in a neutral stress field. This phenomenon was clearly observed, especially under a low stress intensity factor condition.