Carbon nanotubes (CNTs) were chemically drilled by supporting cobalt oxide in a controllable way. A series of drilled CNTs with different Co loadings were prepared, characterized and used as the oxygen reduction reaction electrocatalysts. The results show that for the drilled CNTs with the cobalt loading less than 8 wt%, the defects or edge carbons as dominating factor increase the electrocatalytic oxygen reduction reaction (ORR) activity; for the drilled CNTs with the cobalt loading more than 12 wt%, the electron transfer resistance becoming the main determinant decreases the ORR activity. The properly drilled CNTs show dramatically enhanced ORR performance than the original CNTs. (C) 2016 Elsevier Ltd. All rights reserved.