Abstract: A new technique of structural dynamic analysis especially for analysis of gear drive system has been developed,which combines the principle of substructure analysis with the idea of structure modification.Meanwhile the gear stiffness model about the de grees of the freedom of the structure node in which the gear relation exists has also been established.It has the advantages of both the method of substructure analysis an d vibration reanalysis of modified structure.The natural frequencies and modes calculated by the program written by author are in quite good agreement with the results calculated by finite element method.