In this study, the chemical, thermal and diffusion effects of H-2 and CO addition on the characteristics of methane laminar flame are examined numerically by using the CHEMKIN 2.0 code with a modified GRI-Mech 3.0 mechanism. The results reveal that a better agreement between the measured and predicted laminar flame speed is obtained by using the modified mechanism. The effect of H-2 addition to flame speed is mainly due to chemical effect at lean and stoichiometric conditions. However, with the addition of CO, around 75% of the increase in laminar flame speed is due to thermal effect. Soret diffusion of H have an obviously effect on laminar flame speed, lowing it around the peak segment. Furthermore, the combined Soret effect of H and H-2 basically accounts for the total Soret diffusion effect. With the addition of H-2 and CO, the lean flammability limit is extended to a leaner mixture at atmospheric pressure. A linear correlation between the laminar flame speed and the relative amount of H-2 was found in tertiary CH4/CO/H-2 mixtures. However, this linear correlation is undermined when CO mole fraction is over than 45%. Copyright (C) 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.