Interfacial reactions between an Al thin film and a single-crystal (001) 6H–SiC substrate were investigated using x-ray diffraction and cross-sectional transmission electron microscopy. Aluminum thin films were prepared by radio-frequency magnetron sputtering method on 6H–SiC substrates at room temperature and then annealed at various temperatures from 500 to 900 °C. A columnar-type polycrystalline Al thin film was formed on a 6H–SiC substrate in the as-deposited sample. No remarkable microstructural change, compared to the as-deposited sample, was observed in the sample annealed at 500 °C for 1 h. However, it was found that the Al layer reacted with the SiC substrate at 700 °C and formed an Al–Si–C ternary compound at the Al/SiC interface. Samples annealed at 900 °C showed a double-layer structure with an Al–Si mixed surface layer and an Al–Si–C compound layer below in contact with the substrate.