Abstract: The micro-crack of materials, undetcted at the early age, will impair the stiffness and strength of structures, and induce structural accumulated damage. Consequently, it may cause the sudden collapse of structures. Thus, it is of important theoretical and practical significance to identify accurately the early micro-damage of materials. However, it is difficult to detect the occurance and propagation of micro-crack inside structures and determine the micro-stress field at each location. In this paper, the identification technique for micro-damage of concrete/steel structures based on piezoelectric monitoring signals is presented recognizing the advantages of PZT M/E impedance, i.e., sensitive to structural micro-crack and robust to enviroment influence. It is expected to provide a reliable and accurate basis for health monitoring of concrete structures. The recent research on damage detection of structures based on piezoelectric impedance technique in the authors’ group has also been presented, and future research topics of the piezoelectric impedance technique have been pointed out.