The structure of niobium oxides was investigated by using XAFS and surface science techniques. The origin of acidic property and catalysis of niobium oxide is discussed by these results. From the EXAFS analysis, the structure of highly acidic hydrated niobium oxide is different from those of T-Nb2O5 and K8Nb6O19, which has been previously demonstrated as the structure of hydrated niobium oxide, because Nb-Nb and Nb-O distances are significantly different from each other. There are at least two types of Nb-O bonds, a tetrahedral form and an octahedral form are existing for hydrated niobium oxide. The excess of negative charge is present for Nb-O bond by applying Tanabe’s hypothesis to single metal oxide consisting of at least two metal-O forms with different coordination number. That seems to be the origin of the acidity of hydrated niobium oxide. The ordered niobium oxides thin film on Pt(111) and their defect face were prepared by using UHV surface analysis apparatus, The oxygen vacancies of niobium oxide on Pt(111) were created by Ar+ ion bombardment. H2O and CH3OH exhibit associated adsorption on the ordered NbOx and Nb2O5 surface and dissociative adsorption on the defect Nb2O5 surface at room temperature. On the ordered NbOx and Nb2O5 surface, no evidence of ethene adsorption at 200 K is found by HREELS, although ethene adsorbs molecularly on the defect Nb2O5 surface with a pi-complex formation. These results imply that oxygen vacancies of NbOx surface play an important role on the adsorption of the molecules.