Shaking table tests are carried out to investigate the nonlinear oscillation of a floating roof in a cylindrical tank under liquid sloshing. The experimental tank is a stainless steel cylinder of 800 mm in radius with a single-deck type floating roof composed of an inner deck made of polyvinyl chloride sheet and an outer pontoon made of acrylic plate. The razor sensor system is employed to measure the roof displacements. The tests are performed using the 2003 Tokachi-oki earthquake record at

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... akomai K-NET station. The time scale of the earthquake record is so reduced according to the similarity law that the test model corresponds in real scale to a typical oil-storage tank of 30,000m 3 capacity. It is observed that the nonlinear oscillation modes with the numbers of circumferential waves 0 and 2 may occur, which can never be predicted by the linear theory. The test results are compared with nonlinear analysis based on the hybrid analytical and FE method proposed by the authors. Overall agreement is confirmed between the analysis and the experiment.