Niobium oxide electrochromic thin films were prepared by reactive d.c. magnetron sputtering, and their physical properties, such as surface structure, composition, optical transmittance, reflectance and absorption, were studied. The surface morphology of the heated sample is very different from that of the unheated sample. Auger electron spectroscopy (AES) measurements show that the composition of the amorphous film is close to Nb2O5 as well as the crystallized sample. The crystallized and amorphous samples show very different absorption behavior. The band gap is estimated to be 3.41 eV for the crystallized sample and 3.45 eV for the amorphous sample.