We have recently carried out model tests of multi-unit floating structures in waves. Up to 5-unit semi-submersible-type floating units were modeled in scale of 1/80. In the test of wave exciting forces, 1-unit, 3-unit, and 5-unit systems were tested in regular and irregular waves in order to obtain the effects of hydrodynamic interaction among floating bodies on their wave exciting forces. In the test of dynamic responses, 3 and 5-unit floating structures were connected to one or two foundation units settled in the model basin. Structural wave loadings and motion responses were successfully measured in head, beam and oblique wave conditions. Results of the tests were compared with theoretical predictions and following conclusions were obtained. (1) A theoretical method, previously proposed by authors, can estimate wave exciting forces accurately including the interaction effect. (2) The elastic structural responses of multi-unit systems can be estimated well using the above mentioned theoretical wave exciting forces.