Abstract: The reflection and transmission of Lamb waves at overlap joints are researched by the numerical method and the experiments. The numerical method is used to simulate the reflection and transmission occurred when the Lamb waves SO and A0 mode (at an exciting frequency of 500 kHz) propagate through welds with different widths on a 2 mm-thick stainless steel plate. The reflection and transmission coefficients are calculated as well. When the welds widths are less than 4 mm, a quasi-linear correlation is observed between the reflection coefficient of SO mode and the weld width. In contrast to the weld width, both the gap between two plates and the reinforcement of weld do not have obvious effect on the reflection coefficient. Based on the results, we propose that the width of weld could be rapidly measured through examining the echo amplitude of SO. Experiments using welds with different widths on a 2 mm-thick stainless steel plate further confirms the quasi-linear correlation and the feasibility of our proposed method.