A partially oxidized $TaC0.58N0.42$ has been investigated as a new cathode for polymer electrolyte fuel cells. The catalytic activity for oxygen reduction reaction (ORR) significantly depended on an oxidation state of the $TaC0.58N0.42$. $TaC0.58N0.42$ and $Ta2O5$ had a poor catalytic activity for the ORR. The onset potential on the partially oxidized $TaC0.58N0.42$ for the ORR had a maximum value of 0.9 V versus reversible hydrogen electrode in $0.1 mol dm−3$$H2SO4$ at $30°C$, which indicated that the partially oxidized $TaC0.58N0.42$ had definite catalytic activity for the ORR. Hard X-ray photoelectron spectroscopy and X-ray diffraction analyses revealed that the surfaces of the partially oxidized $TaC0.58N0.42$ were oxidized, although the inner parts of the partially oxidized $TaC0.58N0.42$ remained tantalum carbonitride. These results suggested that an appropriate oxidation of the $TaC0.58N0.42$ was essential to enhance the catalytic activity for the ORR.