Nanocrystalline FeCoNi and FeCoNiSi powdered alloys were prepared by mechanical milling process (MA). Using X-ray diffraction patterns, we experimentally proved that when MA reached a time of 50 h, it led to a decrease of the crystallite size down to 20 nm and 32 nm for FeCoNiSi and FeCoNi, respectively. However, the dislocation density increased, reaching the highest value for the alloy associated with silicon. Nevertheless, this high energy ball-milling process is not used only for the refining of microstructure, but also to induce either a chemical reaction between the powdered chemical elements or a phase transformation, such as the allotropic transformation of HCP-Co to FCC-Co and the formation of highly disordered Fe-based solid solutions. Thermal stability of the milled mixtures was investigated by DSC from 25 up to 700℃ at a heating rate of 10℃/min. Various milled samples were first annealed at specific temperatures and then analyzed using X-ray diffraction, which demonstrated the stability of the evolved phases during subsequent heating and the formation of some metallic oxides, such as Fe3O4, Fe2O3 and FeO, particularly for the elevated annealing temperatures.