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In the present paper, a simulation program is developed for multiplefatigue cracks propagating in a three-dimensional stiffened panel structure, where itcan predict fatigue crack lives and paths by taking into account of the interaction ofmultiple cracks, load shedding during crack propagation and welding residual stress.Various fatigue crack propagations in longitudinal stiffeners of ship structures areinvestigated by both the present simulation method and experiments. From these results,it is found that the crack propagation may considerably change, depending on theloading conditions, structural details and residual stress distributions. This means thatone could possibly manage to avoid fatal damage of the skin-plate by properlydesigning the structural details.