The lack of the high temperature I-V model greatly restricts the application of GaN HEMT devices. In this paper, the characteristic variation of GaN HEMT device under high temperature condition is investigated, and an improved I-V characteristics model of GaN HEMT transistors over a wide temperature range from 25°C to 300°C is proposed based on the classic Statz model. The experimental results indicate that the improved spice model, by taking the self-heating effect into account, is more accurate compared to the original Statz model. The proposed I-V model should be an available tool for the simulation of GaN HEMT device in designing integrated circuits at high temperature.