Abstract: Slots with different depth were made by WEDM in metal sheet of ST12 to simulate slot-type defects. The Lamb wave was stimulated by pitch-catch and pulse-echo method. The time domain signals obtained in the defect-free plate and the defected ones, respectively, were analyzed in time-frequency domain by Short Time Fourier Transform. The lamb wave modes and mode conversion were recognized successfully. There was a good corresponding relationship between slot depth and the energy distribution center of the main Lamb wave mode.