Two series of Nb2O5/SiO2 catalysts are prepared by an equilibrium adsorption method (type A) and a conventional evaporation to dryness method (type E). The loading amounts of type A catalysts were low (approximately 0.2 wt% as Nb2O5) and those of type E catalysts were varied from 0.1 to 10 wt%. UVNIS diffuse reflectance spectra of the catalyst samples show that micro particles of Nb2O5 are present on the catalysts with high-loading and monomeric or oligomeric niobate species are present on the catalysts with low-loading. By analyzing luminescence spectra of the low-loading samples, we conclude that the type A catalyst contains monomeric NbO4 tetrahedra and the type E catalyst oligomeric NbO4 tetrahedra. Propene photo-oxidation on type A catalysts yields propene oxide selectively, formed on monomeric NbO4, whereas the photo-oxidation on the type E catalyst with low-loading yields propanal selectively. Propanal is the product in decomposition of propene oxide on oligomeric NbO4 tetrahedra.