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Based on the electro-mechanical impedance technology, experiments were carried out to study the damage identification. A precise impedance analyzer HP4294A was used to extract electricity admittance from free piezoelectric (PZT) patches to analyze the characteristics. And extract and analyze electricity admittance from PZT patch on the steel beam with original undamaged and different depth of cracks. In order to characterize the damaged severity of structure quantitatively, a damage index RMSD was employ in analyzing. The experiment results showed that Electro-mechanical impedance method can feasibly be used to detect small damages, the electricity admittance curves shift toward the left and the resonant frequencies corresponding to peak values reduce with increasing the depth of cracks in the steel beam. Index RMSD could quantify the damage degree precisely and effectively. Further study showed the effect of temperature, the length of the wire and the excitation voltage will be considered in the electricity admittance signal. These experiments lay basis for Electro mechanical impedance technique applied in practical engineering.