The responses of a large floating offshore structure, which has quite large length-to-breadth ratio and is considered as a flexible structure, are dealt with in this paper using both experimental and theoretical approaches. The experiments were carried out in regular waves, long-crested and short-crested irregular waves with two different types of flexual rigidity. According to the results of experiment, it is found that the flexibility of structure affects strongly the characteristics and amplitudes of motion response as same as deflection. The transfer functions of these response are calculated based on modal analysis using ‘wetted mode’. Also the influence of mooring system and directional distribution of irregular waves are discussed. The estimated response spectra in irregular waves and statistical values which are predicted from the response spectra are compared with measured one. Finally, the vertical motion characteristics along longitudinal direction in waves are investigated from viewpoint of deign of such floating airports, some studies and works necessary in future are proposed.