Temperature distributions in AlGaN/GaN high-electron-mobility transistors under bias voltage have been measured by micro-Raman scattering spectroscopy. The temperature was estimated from the Raman shift of E2 phonons of GaN. The spatial and temperature resolutions are 1 µm and 10 K, respectively. When the power dissipation was 248 mW at a drain voltage of 40 V, the peak temperature of 443 K was observed at the gate edge on the drain side at the center of the channel. This position corresponds to the high-field region at the gate edge.