We present the Rf characteristics of 0.7-μm gate length n-GaN/AlGaN/GaN high-electron mobility transistors (HEMTs) with different source-drain spacing tested under different temperatures. The 7-μm source-drain spacing device demonstrated 800 mA/mm drain current density and 257 mS/mm tranceconductance, and the 5-μm source-drain spacing device demonstrated 700 mA/mm drain current density and 260 mS/mm tranconductance. The 7-μm source-drain spacing device was measured at room temperatures of 25 °C and -40 °C, the current gain (fT) were 18 GHz and 21GHz and the maximum oscillation (f max(U)) frequency were 63 GHz-and 87 GHz, respectively The f T was nearly linearly dependent on the temperature. As operating temperature increased from -40 °C to 50 °C, the fT dropped more dramatically for the 5-μm SD spacing device than for the 7-μm device. The fmax characteristic of 5-μm SD spacing device decreases more dramatically above 125 °C than the 7-μm SD spacing device. This phenomenon might be due to stronger phonon scattering for shorter channel device at high temperatures.