The generation of the longitudinal waves in the frequency range up to the microwave region was observed in the GaAs Gunn diode when the applied field exceeded the threshold. This ultrasonic excitation in the Gunn diode is caused by the lattice deformation by the high‐field domains through the piezoelectricity of GaAs. The two possible excitations, in the bulk and at the surface, are discussed using simple models. It is speculated that a properly oriented Gunn diode may be applicable to the narrow beam‐like ultrasonic generator.