One of the causes for the crack damage on the stiffened bulkhead in the neighbourhood of the joint with stiffeners is the fatigue fracture due to the vibration of the plate accompanied by the torsional and bending vibration of the stiffeners. The basic equation representing the stiffened plate which is freely vibrating are introduced by combining the plane stress theory and the equation of lateral vibration of plate, and the dynamical stress distribution of stiffened plate is analysed in the state of its free vibration to make this cause clear. The eigen-values and dynamical stress distributions of the simply supported rectangular plate stiffened by a inverted angle at its centre line are investigated theoretically and experimentally. The experimental results agree fairly well with theoretical ones. The dynamical bending stress in the plate produced in the neighbourhood of the intersection of stiffener and plate are considerably large. Assuming the fatigue limit as σ_F= 22 kg/mm², at the time when the maximum dynamical bending stress of the vibrating stiffened plate fall on that fatigue limit, the ratio of the maximum deflection of the plate to the stiffener space is about 2× 10^-21 × 10^-3 for the 1 st3rd mode. But the assumed fatigue limit is the value under an ideal condition, and therefore, under an actual condition, this value will decrease to 1/2 1/3, provided that the shape factor as well as the change in quality of materials due to welding and the influence of corrosion are considered. Thus it is conceivable that the fatigue fracture of the stiffened plate may be caused by local vibration of ship.