In the usual analysis of floating structures, rigid body motion is analized first and afterthat, global structural analysis as a elastic body is conducted. For large floating structures, however, elastic deformation of structural members may affect the motion of them. In this paper, a method analyzing the motion of floating framed ocean structures subjected to periodic waves is presented, where flexibility of members is included and internal stresses of them are calculated simultaneously. In this method, functions of subdivided elements of members are assumed to be divided into two. One is a rigid body with mass, shape and sizes and the other is a elastic beam with elastic rigidity. The equations of motion are introduced by using matrix method. The calculations based on the mentioned analytical model were compared with experiments by simple floating models and the coincidence was fairly good.