Silicon‐deposited Ni tips were heated at various temperatures up to 1000 K and structures and compositions of thin Ni silicide layers formedinsituwere investigated by the atom probe and the field ion microscope (FIM). Although a mixed layer of Ni and Si was formed even at 80 K, FIM images of an ordered silicide structure were observed after heating the tip above 900 K. It was noticed that the evaporation field of the ordered silicide is significantly higher than those of Ni and Si, possibly due to a strong binding force among the constituting atoms. Since Ni is the major moving species, the Ni concentrations in the thin silicide layers were found to be as high as Ni4Si near the surface and decreased to NiSi at the boundary of the silicide and the substrate Ni. Although the composition of the silicide, which showed the ordered image, changed from Ni5Si2to NiSi as the mass analysis was proceeded from the surface of the silicide to the substrate Ni, the silicide consistently exhibited the FIM image which was characteristic of a cubic structure. The computer‐simulated image of the hypothetical structure for the observed silicide, a modified cubicC1 structure, showed the characteristic features of the observed silicide image.