Recently, various kinds of arctic offshore structures have been constructed corresponding to the promising oil industry. In designing such structures, the designers have to pay particular attention how to overcome the ice loads. One of solutions for strength against ice loads is the application of composite steel-concrete structure. In this paper, selecting five feasible composite models where concrete is placed inbetween steel plates, the elastic-plastic behaviour and their ultimate strength are investigated by both theoretical and experimental approaches. The theoretical analysis has been performed by finite element method which is newly developed to incorporate non-linearity of concrete and interaction between steel and concrete. In addition, thermal fatigue, i. e. recurrence of freezing and thawing of concrete, is experimentally investigated anticipating real environmental condition in arctic area. As the results of our investigations, followings can be concluded. 1) Our newly developed computer program can predict the elastic and plastic behaviour and ultimate strength of composite structure with due accuracty. 2) Most benefitial combination of steel and concrete can be found. 3) The recurrence of freezing and thawing do not give an appreciable influence on the strength of structure.