The outward diffusion equation of the instantaneously released ion in pores of a porous layer was derived theoretically, and then applied for the study of oxygen ion diffusion in porous Fe which was generated by fast electrochemical reduction of solid Fe2O3 in molten CaCl2. The solid cathode was constructed by filling mixed Fe/Fe2O3 powders (8:1 in mass ratio) into cylindrical cavities in a Mo foil substrate. This fabricated mini porous electrode was subjected to the double-potential treatment with selected potentials according to the cyclic voltammetry (CVs) of Fe2O3. The lower potential of −0.7V (vs. quartz sealed Ag/AgCl) enabled fast reduction of solid Fe2O3, and instantaneous release of O2− ions into the pores of the generated porous Fe. During the standing time at −0.7V, outward O2− diffusion occurred, and the amount of O2− remaining in the porous electrode was determined by re-oxidation at a potential of 0.16V. The experimental outward diffusion of O2− was in good accord with the theoretical equation, consequently, the diffusion coefficients of O2− in CaCl2 contained in porous Fe were evaluated as 8.2×10−6, 9.1×10−6 and 1.0×10−5 cm2 s−1 at 1108, 1123 and 1138K respectively. These data followed the Arrhenius’ equation with diffusion activation energy of about 67.8kJmol−1. The whole work can provide a simple method for the study of diffusion in a porous electrode, and the designed ultrafast deoxidation of solid Fe2O3 may be also helpful to the present effort in developing iron metallurgy by molten salt electrolysis.