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The properties of organic electroluminescence devices are largely affected by the kind of metals used as the cathode, because the barrier height formed at the interface depends on the electronic and chemical properties of the metals. On the basis of the temperature dependence of currents flowing through the interface between tris(8‐hydroxyquinolino)aluminum and a magnesium cathode, the carrier injection mechanism at the interface was ascribed to the thermionic emission with the barrier height of about 0.58±0.03 eV at zero electric field. The barrier height became larger when metals with larger work functions were used as the cathode. The current density was also found to be dependent on the chemical structure of the interfaces.