© 2020 Elsevier B.V. The direct integrating of GaAs and diamond is achieved at room temperature via a surface activated bonding method. An ultrathin crystal defect layer composed of GaAs and diamond was formed at the bonding interface. The thickness of the GaAs and diamond crystal defect layers was determined to be 0.4 and 1.6 nm, respectively. After annealing at 400 °C, no changes were observed in the thickness of the crystal defect layer and the interfacial structure. The thermal characterization of the transmission line model (TLM) patterns formed on the GaAs layer bonded to diamond and sapphire substrates is demonstrated. The thermal resistance of the GaAs TLM patterns formed on the diamond and sapphire substrates was determined to be 6 and 34.9 K/W, respectively. The GaAs TLM patterns formed on the diamond showed an excellent heat dissipation property due to the high thermal conductivity of diamond.