Fundamental structural response characteristics of a typical semisubmersible in waves are investigated both by model tests and theoretical calculations. A 1/50-scale structural dynamic model of 2-lowerhull and 8-column type was made of acryl and the wave tank tests were carefully carried out. The platform motion, mooring tension and the axial and bending stress of the braces and deck girders were measured in detail. The measured wave-induced responses were compared with three-dimensional motion and frame structural calculations. In this paper stress is put on the axial force responses of major structural members. Structural response characteristics are also studied through simplified estimation of wave loads. As a result, the experimental responses agree with the calculated ones fairly well except for a particular brace, and the fundamental structural dynamic response characteristics of this type of semisubmersibles are confirmed both experimentally and theoretically. The detailed experimental results presented in this paper are expected to be valuable for the study and design of similar types of semisubmersibles.