Thin layers of niobium oxide (NbOx) were accumulated by the sol-gel process, with the sol of NbOx in ethanol prepared by partial hydrolysis of commercial niobium(V) ethoxide, on glass plates coated with transparent conducting tin oxide. Characterization by x-ray diffraction, differential thermal analysis, and thermogravimetry revealed that the as-prepared film, consisting of fully hydrated amorphous niobium(V) oxide, undergoes dehydration into the partially hydrated form and, finally, crystalline niobium(V) oxide (Nb2O5) by calcination at the temperature up to 873 K. The films exhibited electrochromic (EC) properties; the reversible color change was observed between colorless and brown-black by alternating anodic and cathodic polarizations, respectively. Among the films used in this study the crystalline Nb2O5 film showed the best EC properties and its spectral change, durability for repeated coloration-decoloration cycles, and retentivity of colored states, i.e., memory characteristics under open-circuit conditions were investigated in detail.