Polyacrylonitrile (PAN)-based carbon nanofibers prepared by electrospinning were physically activated using carbon dioxide as the oxidizing agent. The activation procedure was performed at 800 degrees C for different periods of time ranging from 15 to 60 min. The activated materials have a hierarchical structure with two sets of pore systems in the micropore range centered at similar to 0.8 nm and small mesopore range centered at similar to 2.8 nm. The activation not only increased the specific surface area and pore volume to 1123 m(2) g(-1) and 0.64 cm(3) g(-1), respectively, but also resulted in the evident loss of doped N atoms. The pyridinic and graphitic nitrogen groups are dominant among various N functional groups in the activated samples. CACNF-60, prepared by activating the carbon nanofibers (CNFs) for 60 min, showed excellent electrocatalytic activity for the oxygen reduction reaction (ORR) as well as superior long-term stability and methanol tolerance compared to commercial Pt/C in alkaline media. The excellent electrocatalytic activity of the activated sample is mainly due to its high N content (6.9 at%), unique hierarchical micro-/mesoporosity, and large specific surface area.