Abstract: Guided waves are used in structure health monitoring. However, the signal will be too complicated to distinguish the damage as multi-modes propagating in the wave-guide. Single mode only is therefore needed in actuating the guided waves. The surface bonded piezoelectric wafers are used as actuator and sensor in this paper. Based on the experiments, the excitabilities of different modes are studied. The results show that not only the S0 mode, but also the A0 mode will be stimulated by the surface bonded wafer. In order to inhibit the A0 mode, the diameter of the wafer should be lager than the wavelength of the mode.