Guided waves have the potential to recognise the corrosion damage in reinforced concrete (RC) at its nascent stages. The latest approaches to generate guided waves deploy contact acoustic transducers and piezoelectric wafer transducers (PWTs). Experimental studies show that excitation of guided waves using contact transducers causes multiple longitudinal guided wave modes in embedded rebars and these modes can be used to assess various features of the damage. In contrast, the excitation with piezoelectric transducers introduces only a dominant wave mode. In this study, mode selectivity of guided waves in embedded rebars is investigated. Finite element simulations are adopted to calculate the response of the RC beam to PWT excitation and are verified with the experimental signal. The frequency dependence of the dominant guided wave mode is studied by examining the displacement profile of the RC beam. The displacement pattern is compared with the mode shape of various wave modes that are calculated using theoretical dispersion curves to recognise the possible wave modes. It is found that the lower frequencies cause F(1, 1) mode, whereas the higher frequencies trigger higher-order flexural modes. The results of this work assist in selecting the appropriate frequency to assess damages of debonding or diameter reduction.