The reaction characteristics of combined steam and carbon dioxide reforming of methane (CSCRM) were examined using a Ni/Al2O3 catalyst for the gas-to-liquid (GTL) process. The concentrations of the products (outlet H-2 and CO) were affected by the concentrations of the reactants (CH4, H2O, and CO2) in the feed, which also influenced the catalyst activity based on the quantity of carbon deposited on the catalyst surface. Scanning electron microscopy (SEM) and thermogravimetric analysis (TGA) showed that the quantity rather than the type of carbon deposited differed under each reaction condition. It was found that the Ni/Al2O3 catalyst combined with porous membrane reactor can efficiently enhance the catalytic activity, where the methane conversion reached 96% with outlet H-2/CO ratio of 2.1 at 800 C (CH4/H2O/CO2 = 1/0.75/0.25) and gas hourly space velocity of 15,600 h(-1) under 2 atm.