Although a continuous reduction of Pt loading further lowers the cost of polymer electrolyte membrane fuel cells (PEMFCs), it increases the oxygen transport resistance within cathode catalyst layers (CCLs) and in turn undesirably sacrifices the cell performance. Tailoring the structure of CCLs via pore-forming has been frequently utilized to enhance oxygen transport, however, the mechanism of enhanced oxygen transport has yet not been fully understood. In this work, magnesium oxide is employed to modify the structure of CCLs, and then the local and bulk resistances as well as the oxygen effective diffusivity (D-O2(eff)) are experimentally measured using a dual-layer CCL design. It is found that after pore-forming, the bulk resistance significantly decreases. Since the experimentally tested D-O2(eff) is two orders of magnitude lower than the calculated value from classical Bruggeman approximation, a modificatory expression of D-O2(eff) is thus given by defining an effective porosity (epsilon(eff)). The extremely low epsilon(eff) (2.14%-3.22%) implies that a highly efficient strategy for ultra-low Pt PEMFCs lies in the design of appropriate electrode structures rather than increasing the porosity blindly. (C) 2019 The Electrochemical Society.