The structural evolution and stability of Fe_ 100-x Ni_x(x=10, 20, 35, 50) alloys prepared by mechanical alloying were investigated through X-ray diffraction analysis and transmission electron microscopy. The intrinsic conditions of preparation determining phase stability in nanocrystalline were clarified. After being milled for 120h, the powders of Fe_ 90 Ni_ 10 and Fe_ 80 Ni_ 20 consist of a single α(bcc) phase, Fe_ 30 Ni_ 30 powders are a single γ(fcc), and for Fe_ 65 Ni_ 35 powders there is co-existence of α and γ phases. The as-milled Fe_ 80 Ni_ 20 powders annealed at 680℃ exhibits the stability of high-temperature γ phase at room temperature, which is consistent with the theoretical prediction. The structural evolution and stability of Fe 100-x Ni x (x = 10, 20, 35, 50) alloys prepared by mechanical alloying were investigated through X-ray diffraction analysis and transmission electron microscopy. The intrinsic conditions of preparation determining phase stability in nanocrystalline were baked. After being milled for 120h, the powders of Fe_ 90 Ni_ 10 and Fe_ 80 Ni_ 20 consist of a single α (bcc) phase, Fe_ 30 Ni_ 30 powders are a single γ (fcc), and Fe_ 65 Ni-35 powders there is co-existence of α and γ phases. The as-milled Fe_ 80 Ni_ 20 powders annealed at 680 ° C exhibits the stability of high-temperature γ phase at room temperature, which is consistent with the theoretical prediction.