Indium tin oxide ͑ITO͒ thin films have been proposed as diffusion barriers for ultralarge scale integrated microelectronic devices. High-resolution transmission electron microscopy and electron diffraction showed that in the Cu/ITO/Si film, the 10 nm thick nanocrystalline ITO film layer works effectively as a barrier. Transmission electron microscopy, scanning electron microscopy, sheet resistance measurement, X-ray diffraction, and energy dispersive spectroscopy analyses revealed that ITO was found to be a good diffusion barrier against Cu at least up to 650°C. The failure temperature of ITO films diffusion barrier ͑10 nm͒ was 700°C. Our results show that ITO film can be considered as diffusion barriers for Cu metallization.