At present, pipeline inspection by electromagnetic acoustic transducers (EMATs) uses amplitude change of the reflection signals from defects. We here intend to develop a more reliable and quantitative method for pipeline inspection, relying on the group‐velocity change of the shear‐horizontal (SH) plate modes, which occurs when they impinge on defected area. The group velocities of SH modes highly depend on the plate thickness, and there exists the cut‐off thickness, below which the corresponding mode cannot propagate. For this, the propagation time of the SH wave will change depending upon the depth and the length of the flaw, when it travels though an affected area. The experimental results showed that the group velocity of SH modes varied with the depth of the defects, and when the thickness became thinner than the cut‐off thickness, it suddenly decreased because of the mode conversion from a higher SH mode to a lower one.