Abstract: Sillicon nanocrystals embedded in a-SiO_x:H (0&It;x&It;2) glassy thin film was prepared by PECVD technique and high-temperature annealing treatment. Using TEM technique, Raman scattering and photo-absorption measurements, the optical properties of the thin films was investigated as well as their structures. The results show that the structure of nc-Si partical is polycrystalline, and the average diameter of the Si nanocrystals increases with annealing tempereture increasing. Optical absorption properties of this kind of film was studied. Quantum confinement effect is observed clearly.With decreasing nc-Si size, the blue shift of the absorption edge is found.