Sandwich composites are widely used in structural applications because of their appropriate mechanical properties and low strength/weight ratio. Delamination is common failure mode in these structures that lead to a reduction in strength and stiffness of composite. In this paper, using acoustic emission, initiation and propagation of delamination in sandwich composite specimens was investigated. The specimens were loaded under mode I loading. Then the characteristics of the signals related to different damage mechanisms were specified. The acoustic emission signals were classified based on their frequency ranges. Then the acoustic emission signals were recorded during the test specimens were processed using wavelet transform. Thus the percentage of energy in each components of the acoustic emission signal was specified. Each of these components has a certain frequency range corresponding to a damage mechanism. Thus the percentages of different damage mechanisms in each specimen were specified. The Scanning Electron Microscopy (SEM) was also employed to verify the results which were obtained from acoustic emission and wavelet transform method. The results showed acoustic emission is efficient tool for identification and separation of different damage mechanisms in sandwich structures.