In this study, impact of pyrolyzing atmosphere on the catalytic activity and structure of non-precious metal Co-based catalysts for oxygen reduction reaction in acid and alkaline medium are investigated. These non-precious metal Co-based catalysts are prepared from pyrolyzing carbon-supported cobalt diethylenetriamine chelate (CoDETA/C) in Ar, N-2 and CO2 atmosphere, respectively. X-ray diffraction indicates that metallic Co nanoparticles with different size are present on each catalyst. X-ray photoelectron spectroscopy shows that the Nat.% is CoDETA/C-Ar (1.54) > CoDETA/C-N-2 (1.39) > CoDETA/C-CO2 (0.42), pyridinic-N and amino-N in CoDETA/C-N-2 is the highest among these three catalysts, while pyrrolic-N, graphitic-N and oxidized-N in CoDETA/C-Ar is higher than that in the other two catalysts. Electrochemical activity demonstrated by cyclic voltammograms and rotating ring disk electrode in O-2-saturated acid and alkaline electrolyte shows that pyrolyzing atmosphere has an important effect on the catalytic activity and a maximum catalytic activity is obtained in N-2 atmosphere, followed by in Ar then CO2 atmosphere, in terms of onset and half-wave potentials, ORR peak potential and current, limiting current, number of electrons transferred and H2O selectivity. The increase in catalytic activity may mainly attribute to the enhanced contents of pyridinic-N and amino-N. (C) 2013 Elsevier Ltd. All rights reserved.