Abstract: Three typical thickness variations of plate-like structures are analyzed in this paper, the stepped, linear, and circular thickness variation. The angle is defined as basic variable, and the effects on the mode conversion behavior are investigated by using ANSYS software. The relationship between the reflection and transmission coefficients of turning modes A₀ and S₀ and the angle of linear slope and osculation is obtained. The results show that the transmission coefficient of A₀ mode is decreased with the increasing of the angle, but the reflection and transmission coefficients of the turning modes A₀ and S₀ are increased. The excitation frequency has influence on the mode conversion behavior of the plate with variable thickness. There is certain difference of the reflection and transmission coefficients of the turning modes A₀ and S₀ between the linear and circular thickness variation. These results are significant to analyze the scattered signals of Lamb wave, thus can improve the accuracy of flaw detection of plate-like structures.