Metal nano clusters are of great importance in physical and chemical science. One simple method for obtaining metal clusters consists in the deposition of a metal film on an inert substrate followed by annealing at high temperature. After this procedure, the metal film reduces in small clusters, whose morphology depends on the annealing temperature and annealing time. In this work we present a grazing incidence small angle X-ray scattering (GISAXS) study carried out in situ to understand the nucleation and formation of Ni metal clusters. For this purpose uniform Ni metal films, of different thicknesses, were deposited onto an oxidized Si(0 0 1) substrate, and annealed at different temperatures in the range 400–800 K. Before annealing the samples were characterized by X-ray reflectivity measurements to exactly determine the values of the thickness and of the starting roughness. The GISAXS patterns show a …