We have studied optical properties and electronic structures of zero-dimensional (OD) Si nanocrystals and two-dimensional (2D) Si quantum wells by means of resonant excitation spectroscopy. Resonantly excited luminescence spectra of 0D Si nanocrystals are sensitive to the surface structure. The size dependence of the photoluminescence (PL) properties and resonantly excited PL spectra of SiO2-capped Si nanocrystals indicate that excitons are localized at the interface between the c-Si and SiO2 surface layer. The TO-phonon related structure in resonantly excited luminescence is clearly observed in H-passivated Si nanocrystals. Hpassivated Si nanocrystals show their crystalline nature, while the oxidized Si nanocrystals show their disordered nature. Luminescence properties of Si/SiO2 nanocrystals are similar to those of Si/SiO2 quantum wells. The electronic structures and luminescence properties of Si nanostructures are discussed.