The high energy ball milling of Ni-50 atom % Si elemental powder mixtures was carried out using a planetary mill. X-ray diffraction (XRD) was used to identify the phase evolutions during the high energy ball milling period. The microstructure morphology of the powders milled different time was determined by field emission scanning electron microscope (FESEM). The beginning time of mechanical alloying was determined by back scattered electrons (BSE) images. The XRD patterns showed that the nickel peaks intensity and the silicon peaks intensity obviously decreased with milling time increased to 1 hour. BSE images revealed that nickel and silicon powders were not blended uniformly for 1 hour of milling. It was found that NiSi formed as the milling time increased to 5 hours, simultaneously, the nickel peaks and the silicon peaks almost disappeared. That means the obvious mechanical alloying started from 5 hours of milling. BSE images agreed with the result analyzed from XRD patterns. With the milling time further increased from 10 to 75 hours, the NiSi peaks decreased gradually, at the same time, the Ni2Si peaks appeared and then increased gradually.