Abstract: In this paper, a computer model is developed for the analysis of tall tube-in-tube structures. By recognizing the dominant modes of the structural actions, the tube-in-tube structure can be replaced by an equivalent plane structure with some fictitious elements at the corners of both tubes. The formulas calcularting the axial stiffness and the flexual stiffness of the fictitious corner columns were derived, And a computer program was developed, The theoretical results were found to agree staisfactorily with the results from the model tests.