The formation process of Nb2O5 films by metallorganic chemical vapor deposition using pentaethoxy niobium (PEN : Nb(OC2H5)5) as a vapor source and O2 as reactant gas has been analyzed by in situ ellipsometry at substrate temperatures of 200 to 500-degrees-C, and at O2 flow rates of 0 to 2000 cm3/min. According to the changes in the refractive index and the growth rate of the films, the formation process can be divided into three stages, that is, the nucleation and coalescence, the homogeneous growth, and the surface roughness evolution stages. The refractive index and the growth rate of the films were found to depend on the substrate temperature and the O2 flow rate. The films deposited at 200 to 400-degrees-C had amorphous structures, and those deposited at 450 to 500-degrees-C contained a delta-Nb2O5 phase in an amorphous matrix. The corrosion resistance of the films in a buffered HF solution depended on both the deposition temperature and the O2 flow rate. The film deposited at 450-degrees-C in the absence of O2, showed the highest corrosion resistance against the solution.