Abstract: With the development of GaN device technology, the output power is increased to hundreds of watts or even to kilowatts. However, the specifications and standards for high output power GaN devices, such as ground situation, have not been clearly defined in simulation design and engineering applications. In this paper, the ground situation of high output power GaN device is analyzed, and its importance to the stability of the power amplifier circuit in ultra-high power GaN devices is verified through simulation and experiment. The ground specification (<1 mΩ) in high output power GaN device employment and the long term reliability of the ground metal surface contacting the device substrate are defined.