The interface reaction between a metal layer and a layer of amorphous indium-gallium-zinc oxide was investigated. Oxygen atoms at the interface bond to the metal atoms and form metal oxide. The reaction depends on the annealing temperature and ambient conditions. The thickness of the metal oxide at the interface increased with the annealing temperatures. The reaction relies on the Gibbs free energy for oxidation. Ta, which has low Gibbs free energy formed a 33 nm layer of tantalum oxide at an annealing temperature of 450 °C. The HR-TEM and EDX observation showed that the metal oxide thicknesses were 5, 10, and 33 nm at annealing temperatures of 350, 400, and 450 °C, respectively. The thicknesses obtained with both Ar and oxygen gas were 4, 8, and 21 nm, respectively. The lower oxide thicknesses were attributed to the lower number of oxygen vacancies in the IGZO deposited using Ar and oxygen, which was identified by XPS analysis.