It is sometimes reported that the heated tank structures in engine room are suffered from serious vibration, in particular fuel oil service tank or fuel oil settling tank. That phenomenon is quite strange, since the tank structure is designed so as to avoid resonance with exciting forces. However the reason of that resonance has not always been clarified in many cases. Heavy vibration of tank panels occurred in a heavy fuel oil settling tank in VLCC. The tank panels vibrated at 6.30Hz to 7.12Hz frequencies with 1.13mm of maximum single amplitude, although the natural frequency of tank panel had been designed to be more than 23.3Hz. The reason of the vibration is investigated on the basis of theoretical analysis and FEM analysis, consequently following facts are clarified : 1) In case of multi-tiered of tanks, oscillation of adjacent tank panels, in particular upper and lower panels, caused the pressure increase due to interference. That pressure reduced the natural frequency of panel due to the increased added mass of fluid. 2) The thermal stress due to the tank heating caused compressive stress in tank panel, since the extension of tank structure was restricted by engine flats, transverse and longitudinal bulkheads of normal temperature. The compressive stress reduced the natural frequency of tank panel. 3) The combination of the above two factors decreased natural frequency of tank panel to 1/4 of the original one and induced the resonance between the reduced natural frequency and the exciting frequency of main engine. This investigation can provide the designers helpful suggestions to avoid the vibration problem of multi-tiered and heated tank structures.