Abstract: Taking into account the fact that there is an inevitable influence of the transmitting and receiving transducers on the time-domain second-harmonic signal of the ultrasonic Lamb waves,a time-domain measurement technique of second harmonic has been proposed using mismatch of group velocities between the fundamental and double frequency Lamb waves.When there is a phase velocity matching but group velocity mismatching between the fundamental and double frequency Lamb waves,the second-harmonic signal of the ultrasonic Lamb wave propagation can be separated in the time domain from that generated by the wedge transducers used for generating and receiving the ultrasonic Lamb waves.The present technique uses the integrated amplitude of the time-domain second-harmonic signal that is only produced by the fundamental Lamb wave propagation to quantify the efficiency of the second-harmonic generation. The measurement procedure of the time-domain second harmonic has been demonstrated by an experiment where a fundamental Lamb wave propagates in an aluminum sheet.It is known that,through the measurement technique proposed in this paper,the condition for measurements of the second harmonic of the ultrasonic Lamb waves can be relaxed,and the influences of the wedge transducers on the second harmonic can be deducted completely.The secondharmonic signal measured by the present technique is completely due to the effect of second-harmonic generation of the time-domain signal of the fundamental Lamb waves.