In this study, effect of high pressure and temperature was investigated on flame speed (SL) in CH4-H2 premixed combustion. In the numerical investigation; H-2 molar fraction, equivalence ratio, pressure and temperature were varied in range of 0-1.0, 0.4-1.1, 4-12 atm, 300-700 K respectively. Heat of reaction was calculated for important reactions and their rates were compared at different mixture conditions. Variation in flame speed was correlated with rate of important reactions. The flame speed tends to increase with hydrogen addition. However, the slope of the relation depends upon the equivalence ratio. Pressure highly influences the chemistry and with high pressure radical concentration seems to decrease. Moreover, temperature increases the reaction rate of almost all the reactions. (C) 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.