CO2 reforming of CH4 on Ni(111) was investigated by using density functional theory. On the basis of thermodynamic analyses, the first step is CH4 sequential dissociation into surface CH (CH4 --> CH3 --> CH2 --> CH) and hydrogen, and CO2 dissociation into surface CO and O (CO2 --> CO + O). The second step is CH oxygenation into CHO (CH + O --> CHO), which is more favored than dissociation into C and hydrogen (CH --> C + H). The third step is the dissociation of CHO into surface CO and H (CHO --> CO + H). This can explain the enhanced selectivity toward the formation of CO and H-2 on Ni catalysts. It is found that surface carbon formation by the Bouduard back reaction (2CO = C-(ads) + CO2) is more favored than by CH4 sequential dehydrogenation. The major problem of CO2 reforming of CH4 is the very strong CO adsorption on Ni(111), which results in the accumulation of CO on the surface and hinders the subsequent reactions and promotes carbon deposition. Therefore, promoting CO desorption should maintain the reactivity and stability of Ni catalysts. The computed energy barriers of the most favorable elementary reaction identify the CH4 activation into CH3 and H as the rate-determining step of CO2 reforming of CH4 on Ni( 111), in agreement with the isotopic experimental results.