Pure nickel and gold thin film were vacuum-deposited on (111) silicon single crystals. When Au/Ni/Si or Ni/Au/Si samples were heated to about 550℃ in situ, hexagonal or deformed hexagonal shaped crystallites were formed on silicon substrates. The formation mechanism and composition of these crystallites were determined by using X-ray diffraction, scanning electron microscopy and scanning Auger micro-probe methods. The crystallites were identified as NiSi₂. The crystallites on the (111) silicon plane parallel to the surface appeared as regular hexagons while the inclined crystallites resembled trapezia. The results of Auger spectra and in-depth composition profiles for Ni. Au and Si showed that the NiSi₂crystallites are islands in a matrix of Au-Si eutectic.