Micrometer-to manometer-scale microstructures of porous Si are investigated by using highresolution scanning electron microscope (SEM), optical microscope, photoluminescence microscope and high-resolution transmission electron microscope (TEM) with aid of photoluminescence spectrometer. SEM and TEM observations revealed that the micrometer-scale structures of porous Si changed greatly by changing the anodization conditions. In contrast to those structures, the nanometer-scale structures of porous Si consisted of several nm to several tens nm single crystalline Si particles surrounded by glass structure SiOx(H). Visible photoluminescence of porous Si seems to come from these fine particles. Correlation between particle size and wavelength of photoluminescence was observed. It is difficult to explain, however, the variations in wavelength of photoluminescence by simple electron confinement model in the three dimensional quantum well structure of particles.