Nitrogen (N)-doped carbon materials (NCMs) are a promising catalyst for oxygen reduction reaction (ORR). However, the formation process of N species in NCMs has never been discussed and which type of N species created ORR active sites is still under debate, which restricts the exploitation of high performance NCMs catalysts. Herein, we firstly prepared a variety of N-doped carbon nanotubes (NCNTs-t) with well-controlled N species derived from oxidized carbon nanotubes (OCNTs-t) with well-defined oxidation degree, to investigate the transformation from oxygen-containing groups on OCNTs-t to consequential N species in NCNTs-t and the ORR catalytic role of different types of N species. A linear relationship existed between the oxygen content on OCNTs-t and the N content in NCNTs-t. Meanwhile, the ketone (C-O) and carboxyl (-COOH) groups were favorable to form the pyridinic N, while the hydroxyl (-OH) and epoxy (-C(O) C-) groups tended to produce graphitic N. Furthermore, the pyridinic N was demonstrated to play the major role on the ORR activity, in which the active sites are the carbon atoms bonded to the pyridinic N. (C) 2017 Elsevier Ltd. All rights reserved.