This paper presents an active monitoring method based on Lamb wave and wavelet transform to determine damage locations. The method compares damage signal with reference signal, namely undamage signal, and takes their algebraic difference as a signal caused by damage. In fact, by the analysis of Lamb wave signals recorded before and after damage, the differential signal is a reflect signal from damage which can be regarded as an acoustic emission signal. It is showed that the wavelet transform using the Gabor wavelet effectively decomposes the differential signal into its time-frequency components, and the peaks of the time-frequency distribution near the center frequency of exciting signal indicate the arrival times of waves. By calculation of time delays between the arrival of the differential and exciting signals, the damage localization points can be obtained. Our experimental results on fiber-glass materials which suffer a
delamination defect prove that the damage location is reliable.