The changes in the structure and phase composition of silicon monoxide in the disproportional reaction at high temperature with the formation of the Si nanocrystalline phase have been investigated. The nanocrystalline powder after separation from silicon oxide by powder diffraction and small-angle X-ray scattering has been studied. It has been found that under optimal conditions of the silicon monoxide heat treatment the obtained powder contains the nanosilicon particles of 17-20 nm size in the volume fraction of 40%.