Carbon deposition over a Ni/Al2O3 methanation catalyst was studied emphasizing the effects of operating conditions. A method to quantify the deposited carbon was developed from TG analysis, and samples were also characterized by TPO and TEM. Carbon formation boundaries were presented in a CHO ternary diagram based on thermodynamic calculations. In the carbon-forming region, it is found that both the temperature and H-2/CO ratio are significantly influential factors for the morphology and amount of carbon deposits. The amount of generated carbon varies with reaction time nonlinearly, meanwhile carbon laydown is favored by low pressure and space velocity. Consequently, it provides guidance for the optimization of operating conditions. .