Heavily Nb-doped anatase TiO (TNO) thin films were prepared by pulsed dc magnetron sputtering using an Nb-doped TiO target. The as-grown films exhibit high resistivity whose resistance decreases by 2104-fold upon vacuum annealing. The 40% Nb-doped anatase TiO film shows a low resistivity of 5.7 10-4 cm and a high electron concentration of 3.07 1021 cm-3. Combined in situ x-ray photoelectron spectroscopy (XPS) measurement and density-functional theory (DFT) calculations show that oxygen interstitial (Oint) and Nb interstitial (Nbint) 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.