Si nanocrystals (NCs) consisting of small crystals from 1 to 20 nm were formed by pulsedlaser deposition (PLD) in inert Ar gas and reactive O2 gas. The oxygen content of the Si NCs increases with increasing O2 ambient pressure and nearly SiO2 stoichiometry is obtained when O2 pressure is higher than 100 mTorr. The optical absorption of the Si NCs shows an indirect band transition. Broad PL spectra are observed from Si NCs. The peak position and intensity of the PL band at 1.8–2.1 eV are dependent on excitation laser intensity, while intensity changes and blue shifts are observed after oxidation and annealing. The PL band at 2.55 eV displays vibronic structures with periodic spacing of 97 ± 9 meV, while no peak shift is found before and after oxidation and annealing. The as-deposited Si NCs show a polycrystal structure and crystallinity improves after annealing. Combined with the PL of Si NCs obtained by crumbling electrochemical-etched porous Si layer, the results give strong evidence that the PL band at 1.8–2.1 eV is due to the quantum confinement effect (QCE) in Si NC core while the PL band at 2.55 eV is related to the localized surface states at SiOx/Si interface.