The coupling of an interdigital transducer to ultrasonic Lamb waves propagating in a piezoelectric plate has been calculated using Green’s function method. It is found that when the plate thickness is more than a few acoustic wavelengths, the lowest order symmetric (S0) and antisymmetric (A0) modes are almost equally generated. The slight difference in the velocities of these modes gives rise to the beating effect whereby wave energy transfers periodically between the upper and lower plate surfaces. Experimental measurements performed on aY‐cut,Z‐propagating lithium niobate plate are found to be in good agreement with theoretical predictions. The coupling coefficients have also been calculated using the simpler definitionK2= 2(v0−vm)/v0, wherev0andvmare Lamb wave velocities on a free and metallized surface, respectively. It is found that this definition gives erroneous results if the ratio of plate thickness to acoustic wavelength is greater than 1.