A systematic evaluation of the oxygen reduction reaction (ORR) on nitrogen-doped carbon nanofibers (N-CNFs) has been performed by tuning the properties of the N-CNFs by using chemical vapor deposition. Analysis of the as-synthesized N-CNFs shows that the iron used as the growth catalyst consists of iron carbides, including Fe7C3, χ-Fe5C2, and θ-Fe3C, depending on the carbon activity of the synthesis feed. Furthermore, a relationship between the growth catalyst phase, the N-CNF properties, and the electrocatalytic activity for the oxygen reduction in acidic electrolyte is revealed. The best catalytic activity and selectivity was achieved if the N-CNFs were grown from Hägg carbide, χ-Fe5C2, suggesting that this carbide phase favors the incorporation of active sites into the N-CNFs. Controlling the phase of the iron particles used as growth catalysts is therefore essential for obtaining N-CNFs with a high active site density for the oxygen reduction reaction.