Electrochemical etching method was employed to fabricate porous silicon(PS)film.Experiment results showed that PS film had multilayer structures.Surface layer was nanostructure

other layers were μm-structure layers.Study of physical and chemical structure of PS film showed that PS was composed of nanosilicon and μm-silicon wires surrounded by Si.O.H.F compounds.PS luminescence comes from surface nanostructure layer.No light emitted from μm-structure layers.We proved from experiment that PS luminescence was relevant to quantum size effect.