Millimeter and submillimeter wave bears some specific characteristics，and plays very important roles in the terahertz field，especially in national defense，security，imaging，communication. This article describes the Gunn devices in the terahertz field. The principles, the process and key technologies of Gunn diode are described in detail，including material growth，Gunn diode package，and Gunn source cavity design. The package of Gunn diode and the specific structure of Gunn device cavity are highlighted. The critical cavity parameters are changed，such as the internal size of the cavity，the waveguide dimensions and models, in order to extract high-order harmonic. The ways to improve the frequency and output power are presented.