We examined synthesis gas (H-2 and CO) production from CO2 reforming of methane using a Co/oxidized diamond catalyst to investigate the properties of oxidized diamond as a support material and as a way to avoid carbon deposition on cobalt-loaded catalysts. Oxidized diamond showed a good catalytic performance among various support materials. No carbon deposition was observed when using the Co/oxidized diamond catalyst in CO2 reforming of CH4. In addition, CH4 conversion was maintained for 10 h during CO2 reforming of CH4 over the Co/oxidized diamond catalyst. In the CO2 reforming of CH4, active cobalt species might be formed or deposited carbon would be rapidly eliminated on oxidized diamond-supported catalyst.