It was found that the luminescence of porous silicon originates from its topmost surface layer, which is regarded as an amorphous layer. Results of this study reveal that the structure consists of nano-sized silicon which is randomly distributed throughout the topmost surface layer of porous silicon. The microstructure of porous silicon looks like a quantum sponge. Not any wirelike structure was observed. Porous silicon is a kind of disorder material. The luminescence from porous silicon is most likely to be due to the quantum confinement in this nano-sized silicon clusters.