This paper describes a new technique for measuring junction temperature with high accuracy in accelerated operational tests (junction temperature ≥200°C) based on the measurements of temperature dependence of gate-leakage current on a GaAs FET (gate-leakage method). The gate-leakage current on the GaAs FET is monitored in the accelerated operational tests. Then, junction temperature and thermal resistance are calculated from the temperature dependence of the gate-leakage current, especially from the temperature dependence in high temperature region. We have measured the junction temperature and the thermal resistance using this method in accelerated operational tests, and have confirmed that the junction temperature can be estimated within a range of ten degrees. The results by this method have been compared with data of simulation, and the validity has been confirmed.