The thermal stability and reliability of AlGaN/GaN high electron mobility transistors (HEMTs) on Si substrates with a 2 μm-thick copper interconnection (Cu-INTC) metal were evaluated. The use of metallic copper as a conducting metal has the advantages of higher thermal conductivity, low cost and low sheet resistance. For comparison, traditional gold metal interconnection (Au-INTC) devices were fabricated under the same process conditions. Thermal infrared (IR) microscopy measurements show that the Cu-INTC devices achieved a lower channel temperature (TCHANNEL) than traditional Au-INTC devices with the identical drain current density. It is owing to its low metal resistivity. The typical peak transconductance (gm), output power (POUT), power gain (Gp) and power-added-efficiency (PAE) at 100 °C operation were 87.53 mS/mm, 22.85 dBm, 11.1 dB and 25.9 % for 1 mm gate width Cu-INTC power device and these measured results were better than those of Au-INTC devices. These measured results indicated that the copper metal provides a highly potential for high-power AlGaN/GaN HEMT applications.