In an attempt to obtain carbon catalysts with higher activity for the oxygen reduction reaction (ORR), a mechanochemical (M)treatment was applied to a heat-treated precursor of carbon-nanoshell-containing catalysts at 500 degrees C, where the nanoshell was a catalytically formed, shell-structured carbon material with diameter of 20-30 nm and walls that consisted of hexagonal carbon layers. The precursor was a mixture of cobalt phthalocyanine and a phenol-formaldehyde resin. The carbonized precursor that was mechanochemically treated after the heat-treatment at 500 degrees C showed higher ORR activity, with ORR current density that was five times that of the carbonized untreated precursor. Characterizations of the precursors and the carbons concluded that the enhanced ORR catalytic activity was due to the formation of carbon nanoshells with disordered hexagonal carbon layers located on their surface, and these nanoshells had a higher N/C surface atomic ratio than those of the carbonized untreated precursors. The application of M-treatment to a 500 degrees C-heat-treated precursor was thus revealed to be useful for improving the ORR activity of carbon nanoshells. (C) 2016 The Electrochemical Society. All rights reserved.