A nickel based catalyst, used in industrial steam reforming, was employed in order to investigate its deactivation by coke formation for steam and carbon dioxide reforming. The coke formation was not observed in steam reforming, whereas the extensive carbon deposition occurred in carbon dioxode reforming. In spite of the large amount of coke formation, the deposited carbon did not deactivate the catalyst rapidly. Deposited carbon had a whiskerlike structure with a nickel particle at the end of the growing whiskers. While the formation of coke was increased as the reaction temperature increased, it was reduced above 700-750℃ by reverse Boudouard reaction, which is known to cause the vaporization of carbon.