Carbon formation behavior under CH4-CO2 reaction and through CH4 decomposition and CO disproportionation was investigated over Ni0.03Mg0.97O solid solution, supported Ni/MgO, and NiO-Al2O3 catalysts by means of thermogravimetric analysis (TGA) and transmission electron microscopy (TEM). Ni0.03Mg0.97O showed high resistance to carbon formation in CO2 reforming of methane and the selectivity to carbon formation was much lower than two other catalysts. It is suggested that CO2 plays an important role in the inhibition of carbon formation on Ni0.03Mg0.97O through the activation of CO2 at the interface between small nickel particles and the support surface.