Perovskite (ABO₃) and ruddlesden-popper (A₂BO₄) oxides are typical mixed conducting ceramic membrane materials for oxygen separation from air. In particular, ruddlesden-popper (RP) membrane display high CO₂ resistance despite their relative low oxygen permeation flux compared to perovskite oxide membranes. Element-doping is an important method to improve the oxygen permeability. In this work, the mechanism of oxygen transfer process through one RP ceramic La₂Ni_1-xMo_xO_4+δ (x = 0, 0.025, 0.05, 0.1, 0.2) membranes was investigated. An optimum doping level (x = 0.05) in La₂Ni_1-xMo_xO_4+δ was found. The optimized composition of La₂Ni_0.95Mo_0.05O_4+δ membrane could not only improve surface oxygen exchange reactions, but also promote oxygen ion bulk diffusion through the dense layer. The maximum oxygen flux of La₂Ni_0.95Mo_0.05O_4+δ membrane reached 3.27 mL min⁻¹ cm⁻² at 1000 °C. Furthermore, La₂Ni_0.95Mo_0.05O_4+δ membrane high stability in CO₂ atmosphere. When sweeping gas was switched from helium to pure CO₂, the oxygen fluxes were only reduced by 5% and stabilized at 2.75 mL min⁻¹ cm⁻² at 950 °C. Our results highlight the efficiency of Mo-doping strategy to simultaneously improve the oxygen permeability and stability of A₂BO_4+δ-type oxide membranes.