Dense fluorite-perovskite-type dual-phase membranes of 60% Ce0.8Gd0.2O2-delta -40% Pr0.6Sr0.4Co0.5Fe0.4Nb0.1O3-delta (CG-PSCF0.4N0.1) were prepared successfully via ethylenediaminetetraacetic acid (EDTA)-citric acid method. Subsequently, the effects of B-site Nb doping on the CO2 resistance, oxygen permeability, and rate-controlling step of Ce0.8Gd0.2O2-delta -Pr0.6Sr0.4Co0.5Fe0.5O3-delta (CG-PSCF) were systematically investigated. The results of high-temperature in situ X-ray diffraction under pure CO2 atmosphere, X-ray photoelectron spectra, and oxygen permeation tests indicated that the thermal stability and CO2 resistance of CG-PSCF could be improved significantly via Nb doping. Furthermore, by measuring the distributions of permeation resistances at certain temperatures using the adopted permeation model, the rate-controlling step was determined, and the oxygen permeability degradation mechanism of CG-PSCF was attributed to the increased bulk diffusion resistance caused by phase transition of PSCF and formation of carbonate. All the results demonstrated that Nb doping has a positive effect on the oxygen permeation stability of CG-PSCF, and CG-PSCF0.4N0.1 dual-phase membranes have promising potential for oxy-fuel combustion.