The compressive strength of the bottom girder of ultra large tankers are studied in the present paper. The size increase of tankers results in the relative decrease of plate thickness to other dimensions. In order to provide a design guideline against surmised buckling, the parallel part models of the bottom girder scaled 1/8.7 are experimented by assuming full load condition and the finite strip method is employed for numerical investigation. The following phenomena are observed from experiments and numerical results. When the spacing of the tripping brackets L is less than the critical length L_cr given by Eq. (6), the local buckling of the face bar occurs in accordance to the elastic buckling of the web panel. The lateral buckling is observed for larger spacing. When the local buckling is dominant, the ultimate strength does not depend on the spacing of the tripping brackets. And the initial imperfection of the face bar decreases the ultimate strength, while elastic buckling value of the web panel and the imperfection of the panel do not have pronounced effects. The necessary stiffness of the tripping brackets to prevent lateral buckling may be estimated as in Eq. (16) for the case L > L_cr .