Abstract: Fault diagnosis of composite structure is increasingly of great interests.It is attractive to diagnose damage by means of vibration theory and experimental technique.In this paper,a new Frequency Response Function (FRF)-Modal Energy diagnositic method is proposed for damage detection of composite laminated plates.The method chooses FRF,which is relative sensitive to the damage and can reflect structure dynamic charateristic,as the diagnostic parameter,and lead to concept of damped dissipative energy and calculates the distribution of generalized modal energy of undamaged plate defined in the paper.The relationship between the varition of FRF at different-orderd resonant peak after damage and the modal energy contribute in each element of undamaged plate in given mode is derived by the theory analyses and calculations.Then,the new method is obtanned to detect the small damaged element or area.An important conclusion of this method is that,in the case of small damage,the ratio of each order generalized modal energy contribution in an element or area is approximately equal to the ratio of change rate at each order FRF resonant peak of undamaged plate in the corresponding damaged element or area.Energy distribution diagrams are given for damage analyses.The results of illustrative examples and experiments show that the method proposed by the aushors is effective and feasible.