We discuss the mechanism of efficient photoluminescence (PL) from Si nanocrystals. Luminescence properties of SiO2- capped Si nanocrystals are different from those of H- passivated Si nanocrystals. The size-dependence of PL properties and resonantly excited PL spectra of SiO2- capped Si nanocrystals indicate that excitons are localized at the interface between c-Si and SiO2 surface layer. The TO-phonon related structure in resonantly excited luminescence is clearly observed in H-passivated Si nanocrystals. H-passivated Si nanocrystals show their crystalline nature, while the oxidized Si nanocrystals show their disorder nature. The luminescence properties of Si nanocrystals are discussed.