Abstract: Lamb wave in thin plate is generally excited by using an angle beam transducer. At some frequencies, the group velocity of Lamb wave excited by an angle beam transducer is nearly same as the velocity of bulk wave, so it is not easy to distinguish Lamb wave from bulk wave through the propagation velocity. This paper respectively researches the reflection characteristics of both the Lamb wave S0 mode and the longitudinal wave in step plates by finite element method (FEM) and experiments, when the incident frequency is 2 MHz. Moreover, it has been found that Lamb waves S0 mode are reflected from the step plates and that bulk waves are not. Based on this reflection characteristic, a method of using step plates to distinguish Lamb waves from bulk waves is presented.