of Nb oxide were identified: Nb-(O) (solid solution), NbO/sub x/ (tetragonal), Nb-O delta -form (hexagonal), gamma -Nb/sub 2/O/sub 5/ (monoclinic), and alpha -Nb/sub 2/O/sub 5/ (monoclinic) were formed by heating in air at either atmospheric or reduced pressure; while, NbO/sub z/ (tetragonal), NbO (cubic), and NbO/sub 2/ (tetragonal) were formed only in air at reduced pressure. The transformation relations between the oxides are made clear. At first, the oxidation of Nb occurs through penetration of O atoms into the metallic lattice of Nb. Then the structures of the oxides transform successively by rearrangement of Nb and the O atoms. The transformation delta and up to st r gamma can take place through a relatively small change in the atomic distribution, while the transformation gamma and up to st r alpha occurs through a remarkable rearrangement of both kinds of atoms, passlng two distlnct intermediate stages. The delta -form may be an oxide inferior to NbO/sub 2/. The tetragonal form NbO/sub 2/ has a super lattice derived from the rutile structure. (auth)