Fe, N co-doped porous carbon nanofibers with iron nitride(Fe-x N) nanoparticles encapsulated in Fe, N co-doped carbon shell(Fe-x N@Fe-N-C) were preparated through electrospinning, carbonization and NH3-etching. The FexN@Fe-N-C catalyst exhibits excellent catalytic activity for oxygen reduction reaction (ORR) in both acid and alkaline media. The half-wave potential(E-1/2) in acidic medium can reach to 0.81 V(vs. RHE), while the E-1/2 in alkaline medium is 0.897 V(vs. RHE), which is even higher than that of commercial Pt/C catalyst. After 10000 cycles of accelerated durability test in acid media, the E-1/2 only decreases by 26 mV, demonstrating the excellent durability of Fex N@Fe-N-C. Ion probing and acid. etching experiments indicate that both Fe-N-x sites and FexN nanoparticles contribute to the ORR activity.