Nb-doped TiO2 (TNO) thin films were prepared by sol-gel dip-coating method with Nb content in a wide range of 0-20 at.%. The prepared films were preheated at 400 degrees C and then undertaken by two different post-annealing processes: (a) three times vacuum annealing and (b) multi-round annealing. The designed multi-round annealing was shown to be an effective way to improve the conductive properties of the films, compared to the traditional vacuum annealing process. The minimum resistivity reached approximately 0.5 Omega cm with Nb doping concentration around 12 at.%, and the carrier density increased with Nb-doping concentration until the critical point of 12 at.%, which might be the optimal doping content for our TNO films prepared by sol-gel method. (C) 2011 Elsevier B.V. All rights reserved.