Polyacrylonitrile (PAN)-based nitrogen-doped carbon nanofibers (NCNFs) were fabricated by electrospinning. Their electrocatalytic activity for oxygen reduction reactions (ORRs) in 0.5 M H2SO4 solution was greatly enhanced by surface modification by oxidation in a concentrated H2SO4/HNO3 mixture and then NH3 etching at high temperature. The modification treatment resulted in a unique porous structure, fiber diameter reduction, more exposed edge planes, a higher proportion of pyridinic-N, and thus enhancement of ORR activity. The modified NCNF electrocatalysts are highly promising for ORRs in acidic electrolytes.