This paper presents the results of comparative calculations on structural responses of a typical semisubmersible and the discussion on design waves for brace stresses. A typical semisubmersible of two-lowerhull type is adopted as a full-scale model for the comparative calculations on three-dimensional motion and structural responses. Based on the comparative calculations by eight different computer programs, standard response functions are proposed as to axial and bending stresses of major braces. Characteristic wave loading patterns, which correspond to design waves, are proposed based on the standard stress response functions. Also simplified equations of wave forces on semisubmersibles, which are useful to consider design waves, are derived based on the assumptions of taking account of only hydrodynamic inertial forces. Based on these results, maximum brace stresses during a 100-yr return period are estimated using design wave method, and are compared with statistically estimated values by short-term and long-term predictions. As a result, it is found that design wave method has a tentative ground for practical design of semisubmersibles.