The effect of Nb doping on the formation and diffusion of O vacancies and interstitial Ti in rutile TiO2 are studied by the use of ab initio density-functional calculations. Our calculation showed that the activation energy for the diffusion of O vacancy with Nb doping is higher than that of pure. That owing to suppressive effect of Nb doping on the formation of O vacancy. Different from the effect of Nb doping on O vacancy, both of the formation energy and migration barrier of interstitial Ti increase with the Nb doping. Our calculated results may be one of the reasons why Nb doping can improve oxidation resistance of γ-TiAl.