Abstract: Since the successful development of nanocrystalline alloys by means of annealing amorphous alloys which were prepared by rapid quenching from the melt, the formation mechanism of nanocrystalline microstructures has been one of the most interesting subjects. According literatures, it has been intensively studied on the effect of additive elements in the process of nucleation and growth. In this paper, the formation mechanism of nanocrystalline microstructure was investigated by means of the thermal expand coefficient and GPS (generalize phonon spectra) in various amorphous materials, including invar amorphous alloys and nanocrystalline soft magnetic materials. Those amorphous alloys which are favorable for the formation of nanocrystalline microstructure appear a phonon softening phenomenon, which indirectly indicates that the interaction forces decrease between atomics and these alloys appear a characteristic of structural instability. It is suggested that the structural instability of amorphous alloys increase the nucleation rate which is favorable for the formation of the homogeneous nanoscale grain structure.