The accuracy of thermal resistance will affect the validity of life test for GaN HEMT devices. IR thermal imaging method for thermal resistance test can only apply to devices prior to capping, but it is not suitable for completely packaged devices. Using transient dual interface test method, combination of forwardbiased gate voltage and the point of separation of thermal impedance curves, the electrical measurement for thermal resistance of an imported GaN HEMT device was studied．The test result compared with IR thermal imaging method, and the variation of thermal resistance was investigated by different cold plate temperature and dissipation power. The results show that the accurate thermal resistance test result of GaN HEMT devices with complete package can be achieved using the dual interface thermal resistance test method, and the change of thermal resistance under different test conditions have a certain rule.