Offshore structures should have a high degree of structural safety even under extreme enviromental loadings. The authors have studied a composite steel-concrete structure of sandwich system for the strength members of huge offshore structures, where concrete is placed between steel plates. In the previous papers, the authors carried out both experimental and theoretical investigations into the strength of the composite structure. It was clarified that the composite structure has various excelent properties: The ultimate load-bearing capacity of the composite structure is very high and it can absorb a great deal of energy until failure under any type of loading conditions. In the present paper, the authors carried out both experimental and theoretical investigations into the dynamic toughness of the sandwich composite structures. Experiments were carried out using the two dimensional models of composite structure under high speed loading. A nonlinear analysis was developed to predict the toughness of sandwich beam under dynamic load, where the material non linearities of both concrete and steel with strain rate were taken into account. The nonlinear analysis accurately represented the behavior and toughness of the sandwich beam structure.