Propagation characteristics and the equivalent circuit model of delay lines using Lamb waves excited by interdigital transducers on unpolarized PZT ceramic plates have been studied. Dispersive characteristics of phase velocities of Lamb waves excited by the interdigital transducers are in good agreement with the numerical solution from the dispersive equation of phase velocities developed by Worlton. When the product of frequency times plate thickness is below 1.3×106Hz mm, the zeroth symmetric Lamb wave is strongly excited. The Lamb waves are excited most effectively when the half‐period of interdigital electrode is equal to the wavelength of the Lamb wave. Also, the values of the attenuation coefficient and the reflection coefficient from the edge of the specimen are given. It is shown that the generation of symmetrical‐mode Lamb waves by our transducer is equivalently represented by a thickness mode transducer of half‐wavelength inserted perpendicular to the plate.