Three-dimensional seismic analysis of a gravity-type steel platform, which is proposed for oil production and storage at marginal offshore fields, is carried out. The present structure is characterized by a massive top deck, a six-leg flexible tubular framed jacket, and a wide spread out base tank. The interest for investigation is, therefore, placed on the dynamic interaction of the structure with soils at site. Different modleings are taken for structural parts in view of the accuracy for response analysis. The dynamic substructure method is applied to advantage for evaluating the soil-structure Interaction effectively. Through the numerical computation, some useful informations are derived for the seismic design of the concerned offshore structure.