The eigen-values and dynamical stress distributions of the stiffened plate contained web frames are analyzed theoretically. As a numerical example, the free vibration of forepeak tank bulkhead of 67, 800 tons tanker, which arranged the web and ordinary frame alternately, are treated. The effect of the aspect ratio of panel on the lowest eigen-value is very important, i, e. in the case of the large aspect ratio, both web and ordinary frame vibrate up and down in the same direction (case 1), but when aspect ratio of the panel is small, the web frames vibrate torsionally and ordinary frames make up and down motion (case 2), and when this aspect ratio becomes smaller, both web and ordinary frames vibrate torsionally (case 3). The effect of the bending stiffness of the ordinary frame on the mode shape is hardly recognized for the case 1, but it becomes the important factors for the case 2 and 3. The approximate calculation method of the eigen-value of the stiffened plate contained web frames is introduced, and the calculation results agree fair well with the exact solution. The considerably high dynamical bending stresses concentrate at each joint of the plate and stiffeners.