The reaction mechanism for CO2 reforming of CH4 on tetragonal ZrO2(101)-supported Pt4 is investigated using the density functional theory method with the periodic slab model. We search and explore two elementary reaction pathways for CO2/CH4 reforming and finally determine the most energetically favorable route through potential energy surface analysis. The key intermediate and rate-determining step are also identified. Our results indicate that the ZrO2 support plays an essential role in the activity of the catalyst in the CO2/CH4 reforming reaction and that it provides a unique adsorption site for CO2, the key to the formation of carbonate and formate species, which could not be realized on the Pt4 cluster and some other supports.