The amount of deposited carbon on activated carbons was adjusted by varying the space time and the time on stream. Carbon nuclei formation appeared to occur initially but was terminated soon and then the carbon crystallite growth became dominant. The methane decomposition rate over activated carbons had a nearly linear relationship with the amount of carbon deposited. This suggests that the carbon deposition occurs uniformly and the activity decreases due to pore blocking, resulting in loss of accessibility to the active sites. The rate was also nearly proportional to the surface area for the same kind of activated carbon, which is contrasted with the previous finding that no discernible trend was observed between the initial activity and the surface area among different kinds of the activated carbons.