Higher modes of SH (shear horizontal) guided wave cannot propagate in a plate which is thinner than the cut-off thickness because of the dispersive character. When a plate contains such a thinning region, a higher mode is converted to a lower one, causing discontinuous velocity change. For the purpose of developing a novel nondestructive evaluation method based on the velocity change, the mode conversion in aluminum plates with thinning region is experimentally explored. Fundamental symmetric SH_0 and the first asymmetric SH_1 modes were generated by an EMAT (electromagnetic acoustic transducer) and propagated in the plates containing artificial thinning regions. The propagated SH waves were picked up at various locations with a piezoelectric pinducer, which is capable of broad range acoustic detection. This experiment observes directly that the generated SH, mode converts to SH0 mode when it enters the defected area, and it converts back to the SH_1 mode after passing through the defect.