It is a well known fact that the fundamental causes of most failures in composite structures are in the forms of incipient damages such as delaminations and cracks which usually remain undetected until they grow to levels large enough to cause failure. In this study, unidirectional carbon/epoxy composite plates with known defects are investigated. The known defects are generated by impacting the composite specimens simulating external collision. A pair of transmitter and receiver was used for generation of Lamb waves and reception of signals. The received signals were monitored by scanning the receiver toward internal defect or by scanning both transmitter and receiver with confined distance over the surface of the composite plates which have known defects lie beneath them. The proper selection of incident angle and frequency are also considered. The characteristics of received signals such as amplitude, energy and wave form are analyzed. The acquired information is used to locate and to measure the size of the impact damage. The suggested method is very effective if the internal damage is presented closed to surface of the plate where the conventional pulse-echo method has problems. The proposed technique can be used widely for the real time and online monitoring of composite structures.