Semi-submersible type offshore floating structures are expected to be used in the Japanese coastal area and at sea off Japan for promoting resource exploitation and development in near future. As the moored offshore floating structures are suffered from current, Vortex-induced Motion (VIM) effect should be assessed in an appropriate manner since the VIM causes fatigue damage of the floating structure's mooring lines. VIM phenomenon on semi-submersible type floating structures, however, is not clear, and its comprehension is insufficient since there are only small number of open studies with lack specifications of the structures. Then this paper represents the results of VIM measurement test using many forms of semi-submersible floating structure models to investigate the effects of column-column interval and lower hull volume for VIM amplitude, and shows the trends of VIM amplitude depending on current velocity and lower hull volume ratio for the first time. Moreover, using these results of the VIM amplitude for the models, fatigue damage of mooring lines is investigated using one sample semi-submersible offshore floating structure.