From the viewpoint of structural safety, leakage of cargo from welded plate structures must be prevented. Fatigue crack initiation and propagation tests were conducted in order to investigate the fatigue crack behavior at plate intersections of outer plate and inner longitudinal web plate and to make clear the outer plate-thickness-penetration condition of fatigue crack from inner members, by using welded plate structure models. In the case of longitudinal structural model with large bracket, a crack initiated from the fillet weld of large bracket and propagated the longitudinal face member and the web plate and then reached the outer plate. The crack did not penetrate the outer plate and propagated along the fillet weld of the outer plate and the inner web plate. On the other hand, in the case of the model with small bracket, a crack from the fillet weld of small bracket penetrated the outer plate. The plate penetration condition was derived on the base of the maximum stress criterion and was verified experimentally. This condition takes account of both the incident crack angle and the primary stress ratio at plate intersections. In the latter case, shear stress was relatively small.