In this paper, we discuss the characteristics of structural response of semi-submersible V.L.F.S. in waves based on rigorous numerical analysis. In this analysis, hydoroelasticity and hydro-dynamic interactions are fully considered. A guide-line for structural design of semi-submersible V.L.F.S. is proposed after relations between the structural response characteristics and structural configuration are discussed. There are two types of semi-submersible V.L.F.S. proposed. One is column-supported type seen in Kansai Intl Airport 1st Phase, floating structure plan. V.L.F.S. of this type is supported on columns of large diameter with small depth. It is shown that the response characteristics can be dominated by quasi-static response pointed out by a part of the present author. The other type is column and lower-hull supported model represented by MOB, whose shape is obtained by extending the present semi-submersible rigs of two lower-hulls. It is shown that some of the most important response characteristics with the structure of this type are horizontal bending in low frequencies and structural resonance.