The photoluminescence(PL)of nanocrystal present in porous silicon shifrs from the near infrared to the ultraviolet depending on the size when the surface is paesivated with Si-H bonds.After oxidation,the centre wavelength of PL band is pinned in a region of 700-750 nm and its intensity increases obviously.Calculation shows that trap electronic states appear in the band gap of a smaller nanocrystal when Si=O bonds or Si-O-Si bonds are formed.The changes in PL intensity and wavelevlgth can be explained by both quantum confinement and trap states in an oxidation layer of nanocrystal.In the theoretical model,the most important factor in the enhancement and the pinning effects of PL emission is the relative position between the level of the trap states and the level of the photoexcitation in the silicon nanocrystal.