A series of nitrogen-doped carbon nanofibers (N-CNFs) have been successfully synthesized via electrospinning N,N-dimethylformamide/polyacrylonitrile followed by KOH activation heat treatment. The electrocatalysts are thoroughly studied with X-ray diffraction, field emission scanning electron microscope, transmission electron microscopy, nitrogen adsorption-desorption isotherms, X-ray photoelectron spectroscopy, respectively, and electrochemical method. The time of KOH activation heat treatment is found to yield a considerable effect on crystallinity, microstructure, chemical surface states, the total content of nitrogen and the active nitrogen configurations in all samples. The resulting N-CNFs-3.0 h catalyst shows remarkable advantages for ORR performance, including porous microstructures, high specific surface area, abundant exposed active sites and high content of ORR-active pyridinic-N. With these favorable features, N-CNFs-3.0 h exhibits an overall distinguished ORR performance in alkaline media compared to commercial 20 wt% Pt/C catalyst, which has a high onset potential of 0.9 V (vs. RHE), half-wave potential of 0.83 V (vs. RHE) and electron transfer number (3.93). The superior ORR of N-CNFs-3.0 h mainly originates from the synergistic effect of porous microstructures and high ORR-active pyridinic-N content, which could be optimized by changing KOH activation time. This work presents a facile strategy to controllably design and fabricate highly active ORR metal-free carbon-based electrocatalysts.