Abstract: An analysis on the failures of floating roof and supporting legs in a 20 000 m³ tank has been made. It is considered that the thinning of the leg-wail is a major cause of the floating roof's failure. Meanwhile, it is pointed out that if the current design principle for supporting-leg is still to be adopted, the failures of floating roof and supporting-legs will not be avoided as yet when a large quantity of oil is unexpectely stored on the floating roof and any 2 pontoons. Therefore)conducting stability checking for the supporting-leg and floating roof and taking effective countermea-sures against the failures, this kind of accident can be avoided.