The success of implementing the mode analysis in the structural integrity research depends to a large extent on the ability to extract normal modes of the structures.In this paper

a new technique is proposed to combine the group theory algorithm and double compatible substructure method for analyzing rotationally symmetric(Cn)structures.In this method

only one sector(1/n part of a structure)is taken and considered so as to obtain the normal modes of the whole structure.But for a large complicated structure

this sector still contains a lot of degrees of freedom(DOF)and is not suitable for direct solution by finite element method.For this reason a substructure method-double compatible mode synthesis method is employed.This technique enhance the extraction efficiency with the accuracy ensured by considerable reduction of synthetic scale through elimination of interface DOF

a numerical example is given to illustrate the merits of the presented technique.