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An ultrathin (thickness 1.0 nm) 2D nitrogen-doped hierarchically porous carbon (2DNHPC) film is developed by the nanocasting method; for comparison, a 3D nitrogen-doped ordered mesoporous carbon (3DNOMC) is also synthesized. Characterizations reveal that 2DNHPC is featured by an extremely high aspect ratio (several hundred) and a bimodal pore distribution. Such a 2D hierarchically porous structure is found to facilitate both the mass transfer of the reactive species and the utilization of active site in the electrode. First, 2DNHPC yields a larger limiting current than does 3DNOMC for the oxygen reduction reaction (ORR), revealing the key role of the low dimensional structure to facilitate the mass transfer. Second, at the loading of 500 μg cm −2 , 2DNHPC shows the same kinetic current with 3DNOMC, indicating that the two catalysts have the same active site and turnover frequency. In comparison, at a lower loading of 250 μg cm −2 , the kinetic current of 2DNHPC remains unchanged, which however seriously deteriorates for 3DNOMC. This result strongly highlights the effect of the carbon dimension on the utilization efficiency of the active site. Finally, it is noted that 2DNHPC yields a comparable ORR electrocatalytic activity and long-term stability with commercial Pt catalyst in both alkaline and acid media. [ABSTRACT FROM AUTHOR]