Heavily Nb-doped anatase TiO2 (TNO) thin films were prepared by pulsed dc magnetron sputtering using an Nb-doped TiO2 target. The as-grown films exhibit high resistivity whose resistance decreases by similar to 2x10(4)-fold upon vacuum annealing. The similar to 40% Nb-doped anatase TiO2 film shows a low resistivity of 5.7 x 10(-4) Omega cm and a high electron concentration of 3.07 x 10(21) cm(-3). Combined in situ x-ray photoelectron spectroscopy (XPS) measurement and density-functional theory (DFT) calculations show that oxygen interstitial (O-int) and Nb interstitial (Nb-int) defect clusters introduce localized shallow p-type accepter states that trap the electrons and reduce the conductivity. These defect clusters can be eliminated by vacuum annealing which is companied by outward diffusion of Nb. As a result, the trapped electrons backfill the Ti sites which are delocalized to promote conductivity.