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With the development of the RF/microwave technology, more and more GaAs PHEMT device is used and the power of the device is also increasing. However, the test of thermal performance and heat dissipation in packaging device is still a prominent problem. In order to obtain steady operation and longer lifetime, what is the highest temperature environment the device can work? Also the package of the device is a major factor of cooling effect, so the measurement of thermal resistance must be accurate. In this paper, some methods was given to measuring of device junction temperature. Traditional test method such as infrared thermography is not work for the packaging device. A new method to measure the thermal resistance of GaAs PHEMT is given. Based on the electrical measurement method for thermal resistance and Schottky junction temperature of GaAs PHEMT and the factors which influence measuring results are investigated. In the meantime we design the setup of the test system for the measuring. The first is drawing the curve of forward voltage changes and temperature. Then the thermal resistance is measured and calculated. The error introduced by thermal resistance of GaAs PHEMT surface is eliminated. The experimental results show that the method has the advantages of simple structure and good stability. The proposed method is capable to evaluate the thermal resistance of the packaging device such as GaN device, VDMOS and so on. It is very significant to provide evaluation method in reliability test.