The phase transition from pure Ni to Ni₂Si and NiSi was studied by in situ Transmission Electron Microscopy analyses from room temperature to 260degC. The reaction starts at 180degC with the formation of small Ni₂Si transrotational domains. Their density and size increase by increasing the annealing temperature to 260degC. After 50 min, a uniform transrotational NiSi layer is formed by a grain by grain transition process. A polycrystalline NiSi layer was instead obtained at 550degC. It was concluded that the final layer structure is the result of a competition: at low temperature (260degC) transrotational domains prevails since slow structural modifications allow the match between silicide and silicon during the Ni₂Si -NiSi phase transition ; at high temperature (550deg), the growth of randomly oriented silicide grains is favoured.