Nucleation and growth of a nanocrystalline cubic NiSe2 phase was performed by mechanical alloying starting from a binary mixture of elemental powders with nominal composition Ni25Se75. The chemical and structural evolution of the mixture was investigated as a function of milling time using X-ray diffraction, differential scanning calorimetry, Raman spectroscopy, energy dispersive spectroscopy and transmission electron microscopy. The results pointed out the occurrence of amorphous selenium, contaminants and/or fluctuations on the local atomic density as the possible reasons for nucleation of a nanocrystalline NiSe2 (Ni33Se67) phase from a Ni25Se75 mixture.