The formation of structures of nickel silicides on Si have been studied by the use of glancing-angle X-ray diffraction, MeV**4He** plus backscattering, reflection electron diffraction, and replica electron microscopy. By reacting evaporated Ni films with Si wafers in the temperature range from 200 to 800 degree C, three Ni silicides have been found. The Ni//2Si phase starts to form at 200 degree C at the Si-Ni interface. Around 350 degree C, the NiSi phase grows from the Si-Ni//2Si interface. The NiSi phase is stable in the temperature range of 350 to 750 degree C and above that it transforms abruptly to NiSi//2. The disilicide grows epitaxially on (111), (110) and (100) surfaces of Si.