For catalytic reactions with simple kinetics the effectiveness factor of catalyst pellet was expressed as a function of the Thiele modulus and the latter was related to the reactant concentration at the center of the pellet. An experimental method was developed to measure the reactant concentration at the center of the pellet as well as the reaction rate and then actually applied to the methanation catalyst that was used in industries. Finally, the experimental results were combined with those of the theoretical study to obtain 0.782 for the Thiele modulus and 0.835 for the effectiveness factor, both at 225 ℃, and 4.75 kcal/mol for the activation energy in the temperature range from 225 ℃ to 300 ℃.