Vibration design of stiffened plates of tank wall is usually carried out in such way that the natural frequencies of both panel vibration and stiffened plate vibration should exceed the exciting frequencies to avoid the resonance. However, panel and stiffener coupling mode of vibration would appear in tank wall adjoining fluid as fundamental vibration mode due to the configuration of tank. If such vibration mode occure, it may cause vibration problem because the natural frequency of this coupling mode vibration is lower than the above two frequencies. In this paper, vibration mode of tank wall filled with fluid is investigated by analytical approach in terms of added mass effect of fluid and the followings are clarified : 1) The natural frequency of panel and stiffener coupling vibration mode “sp” is always lower than that of stiffened plate vibration mode “s”, although it never happens without fluid interaction. The reduction of this natural frequency is caused by the coupling with stiffened plate and small panels due to added mass effect. 2) The natural frequency of panel and stiffener coupling vibration mode “sp” becomes lower than that of panel vibration mode “p” in case that the configuration of the tank satisfies, roughly saying, B / L ≤0.3 or H / L ≥0.5 Therefore, in designing the tank structure, it is important to consider the natural frequency of panel and stiffener coupling mode to prevent vibration problem according to the tank dimension.