Much has been learned about the stress concentration in the vicinity of slots in the ship transverse web frame and the fatigue strength of the slots, contributing greatly to the improvement of ship structural design. The authors produced 1/2 scaled three dimentional models of the transverse web frame members, each containing 9 or 10 slots, and tested them for fatigue strength hydrostatically or by hydraulic-jack mechanical loading. By use of such large models, the authors studied, specifically, (1) the validity of the previous findings made using relatively small models regarding the stress concentration near the slots and strength of the slotted transverse frame against crack initiation, (2) how the crack propagation would be affected by the three-dimensional structural profile and by the presence of adjoining slots, and (3) the fatigue strengths of problem regions of slotted bottom transeverse frames which are to be provided between the outer and inner bottom shells in the projected double-bottomed tanker hull structure, the last named being an important consideration in the prevention of possible sea pollution by leaking cargo oil. Hiroshima Technical Institute and Nagasaki Technical Institute of Mitsubishi Heavy Industries, Ltd., jointly experimented the models, with the former taking charge of the ordinary, slotted transverse “ring” web frame in the tanker wing tank and the later the slotted bottom transverse frames in the double-bottomed tanker. Also, the authors analyzed the crack propagation characteristics using the fracture mechanics technique, the results of which will be reported later on.