Abstract: Based on the modal expansion analysis method, an excitation efficiency parameter is proposed to quantitatively describe the efficiency of accumulative second harmonic generation of Lamb waves. Theoretical excitation efficiency parameters corresponding to the disperse curves of Lamb wave are calculated in a given P92 steel plates. Two types of primary Lamb modes with different excitation efficiency parameters are selected for experimental comparison, which are called longitudinal velocity type S1 mode and intersection point type A2/S2 modes. Then, the second-harmonics generations of the selected two different primary Lamb waves are measured, respectively. The results show that there is a good agreement between the theoretical predictions and experimental verifications of the two different types of Lamb wave modes. It is found that the longitudinal velocity type S1 mode with a higher excitation efficiency parameter corresponds to the higher second harmonies of Lamb wave. Therefore, excitation efficiency parameter developed in this contribution could be an effective indicator for the mode selection of efficient second harmonic generation of Lamb waves.