We have studied the interfacial structures of AlN/Si (111) grown by metal-organic chemical vapour deposition. X-ray photoelectron spectroscopy and Auger electron spectroscopy were used to analyse the components and chemical structures of AlN/Si (111). The results indicated that a mix-crystal transition region, approximately 12nm, was present between the AlN film and the Si substrate and it was composed of AlN and Si3N4. After analysis we found that the existence of Si3N4 could not be avoided in the AlN/Si (111) interface because of strong diffusion at 1070℃. Even in AlN layer Si-N bonds, Si-Si bonds can be found.