The effect of the deflection due to the ununiformity of the temperature distribution caused by the natural environment conditions such as solar radiation cannot be neglected for the design stage of the huge floating offshore platform like a floating airport. In this report, the temperature of the upper structure of floating offshore platform 'POSEIDON' has been estimated although its size of the upper structure is not so large. Further, the distribution of both the deflection and the thermal stress have been obtained by the structural analysis using Finite Element Method (FEM). The following results have been obtained. 1) The central part of the upperdeck has been deflected upward due to the temperature difference between upper and lower side of the box-girder and its deflection becomes largest at noon in a day. 2) The compressed stress occurs in the upper side of the upperdeck. The maximum longitudinal axial stress occurs in the central part of the upperdeck and that of transverse axial stress occurs in the box-girder. 3) The upper structure has been deflected due to the ununiformity of the temperature distribution between side walls even in the horizontal plane. 4) The estimation of the structural strain agrees well with measured results as for the upper side of the box-girder and that as for both side wall and lower side of the box-girder also does qualitatively. 5) The regression equation estimating the maximum and minimum temperature of the upperdeck in a day has been shown using the data of climatic elements in every 3 hours.