It is well known that the fatigue life of welded joints is affected by the size of the specimen. In this study, box-welded rib-plate specimens were prepared and the effects of base-plate size of the specimen on fatigue life were investigated through fatigue tests. Two sizes of specimens, large (width: 150 mm, thickness: 16 mm) and small (width: 40 mm, thickness: 9 mm), were tested and fatigue lives were compared. Results indicate that fracture lives of the large specimens were shorter than those of the small specimens. The fracture life was divided into crack initiation life and crack propagation life. The difference in fracture lives was caused by the crack initiation lives. The crack propagation lives were scarcely affected by the size of the base-plate.