Abstract: The process of connecting two substructures by a component to form a combined structure is dealt with as structure modification in this paper. The known dynamic characteristics of the substructures are used to predict the dynamic characteristics of the combined structure by modal reducing method. The residual adjunct modes of each substructure are added to the keeping ones in order to improve the accuracy. When the substructure has rigid modes, the frequency shifting technique is used. It is shown that if the rigid modes are selected as keeping ones, the computing procedure will be efficient and expedient. Examples are given in the paper.