Tantalum oxide-based compounds were examined as new non-noble cathodes for polymer electrolyte fuel cell. Tantalum carbonitride powder was partially oxidized under a trace amount of oxygen gas at 900°C for 4 or 8h. Onset potential for oxygen reduction reaction (ORR) of the specimen heat-treated for 8h was 0.94V vs. reversible hydrogen electrode in 0.1moldm −3 sulfuric acid at 30°C. The partial oxidation of tantalum carboniride was effective to enhance the catalytic activity for the ORR. The partially oxidized specimen with highest catalytic activity had ca. 5.25eV of ionization potential, indicating that there was most suitable strength of the interaction of oxygen and tantalum on the catalyst surface.