Liquid storage tanks have often suffered buckling failures during earthquakes. In this paper, the buckling of fluid filled cyrindrical tanks under harmonic base excitation is studied using small plastic cyrindrical tanks made of polyester sheet, and two types of buckling mode are observed. The stress resulting from the frequency response (the circumferential wave numbers n=1) is calculated using an axisymmetric shell finite element program including the effect of fluid structure interaction. The static buckling stress criterion for a long cylinder subjected to pure bending is applied to the above calculation. The static buckling criterion proves to be fairly satisfactory in the restricted region under natural frequency of the fundamental mode.