Many floating marine structures of various kinds have been constructed for varied purposes ; however, researches and technical experiences for such structures, some of which are quite different from conventional ships, are not so exhaustive and comprehensive, as those for the ships, and many problems still remain unsolved. Taking an opportunity of construction of newly developed iron ore loading station with submersible, column stabilized, catamaran hulls, a systematic study was made on the motion and strength of the structure in waves. Theoretical analysis of the motion and strength of the structure was carried out first. Secondly, the theoretical results of its motion in waves were compared with that of model tank test. Thirdly, in order to check the theoretical calculation the stress measurements were made at the time of launching. Finally, during the ocean towage of the structure from Japan to India, comprehensive measurements of motion of and stress in the structure in waves were conducted simultaneously with the accurate measurements of waves by means of a clover-leaf buoy. Conclusions of the present study are summerized as follows : 1) Motions of floating marine structure with catamaran hulls in irregular waves can be estimated with sufficient accuracy for engineering purpose by the spectral computations of responses utilizing the linear superposition principle using the response functions which are obtained by the strip method usually used for ship motions in oblique waves and are corrected by the results of model tank test. 2) Longitudinal stress of the structure can be estimated with fairly good accuracy, by the finite element method using the response function of longitudinal bending moment in waves which is also calculated by the strip method.