Electrochemical measurements, including cyclic voltammograms (CVs) and rotating ring disk electrode (RRDE) technique, are applied to determine the impact of surface functional groups on the electrocatalytic activity and H 2 O selectivity of multi-walled carbon nanotubes (MWCNTs) for the oxygen reduction reaction (ORR) both in acid and alkaline electrolytes. Here, three types of carbon nanotubes (CNTs) involving MWCNTs, MWCNTs-COOH (MWCNTs with carboxyl groups) and MWCNTs-OH (MWCNTs with hydroxyl groups) are investigated. Both CVs and RRDE results indicate that the order of ORR electrocatalytic activity and H 2 O selectivity is MWCNTs-COOH > MWCNTs-OH > MWCNTs in acid solution; while in alkaline medium, it is MWCNTs > MWCNTs-COOH > MWCNTs-OH. Furthermore, the ORR on these CNTs proceeds mainly through the two-electron reduction pathway followed by a gradual four-electron transfer reaction. Possible interpretation of the different ORR electrocatalytic activity and H 2 O selectivity in these two solutions is also discussed.