Due to the lack of facilities, only few experimental works have been carried out in the field of motions of a floating body in multi-directional waves. Therefore, the authors carried out experiments on motions of a floating body in two directional waves from rectangle each other in a model basin and they checked the linearity of waves and motions of a floating body. From these investigations, the authors pointed out the limitation of linearity and the interesting phenomena of slow drift oscillation of a moored floating body in two directional waves. They also developed the experimental technique of motions of a floating body in multi-directional waves and derived the theoretical prediction method of these motions in multi-directional waves which is based on functional polynomial method.