Some porous silicon, which were formed on n-type c-Si under illumination during the anodization,show blue photoluminescence after a long period of storage in an atmospheric environment at room temperature. Experiments of photoluminescence and infrared transmission and reflection indicate that samples, which were anodized under strong Ar+ 488 nm laser illumination, have an oxidation layer of better quality, in which there are less stress, less amorphous and a more completely cross-linked Si-O-Si network. And this kind of oxidation is favorable to the blue emission of porous silicon.