Surface structures and photoluminescence (L) mechanisms of porous Si (PS) and the role of oxygen termination on PS have been investigated using a synchrotron-radiation photoemission spectroscopy technique, an Auger electron spectroscopy technique, and a photoluminescence technique. There is almost no trace of oxygen an the surface of as-anodized PS and its surface is covered with hydrides. An increase in PL intensity with increasing anodization current is interpreted to correspond to an increase in the number of photoluminescent elements. The investigation on the oxidation effect suggests that oxygen termination itself does not affect electronic transition levels unless it modifies the size or the structure of the Si crstallites. Energy bandgap analysis indicates that PL peak energy of PS correlates well to the surface energy gap, and that values of energy gaps and exciton energies obtained for PS are close to the results of the recent theoretical calculations for quantum dots and wiles.