Abstract: In order to monitor the damage of integrated composite structures, an energy damage index method was proposed by using Lamb wave and Hilbert transform. First, the envelope of the time signal was picked up according to Hilbert transform of Lamb wave. Then the wavepacket which had the maximum amplitude was selected and the ratio of the wavepacket energy change for damage to the wavepacket energy of baseline was defined as damage index. This index method did not need to select the special Lamb wave mode and was helpful to overcome the problems of signal analysis with frequency dispersion, multi-mode and mode conversion of Lamb wave in composite structures. Finally this index method was tested by monitoring the damage evolvement of composite T-joint. The results show that this method can be applied to the damage monitoring of composite T-joint, when the energy damage index (EDI) value reaches to 0.62, the composite T-joint certainly has been damaged.