Experimental and numerical study on laminar burning velocity of H-2/CO/N-2/CO2/air mixtures was conducted by using a constant volume bomb and Chemkin package. Good agreement between experimental measurements and numerical calculations by using USCII Mech is achieved. Diffusional-thermal instability is enhanced but hydrodynamic instability is insensitive to the increase of hydrogen fraction in fuel mixtures. For mixtures with different hydrogen fractions, the adiabatic flame temperature is not the dominant influencing factor while high thermal diffusivity of hydrogen obviously enhances the laminar burning velocity. Laminar burning velocities increase with increasing hydrogen fraction and equivalence ratio (0.4-1.0). This is mainly due to the high reactivity of H-2 leading to high production rate of H and OH radicals. Reactions R3 and R2 play the dominant role in the production of H radical for mixtures with high hydrogen fraction, and reaction R31 plays the dominant role for mixtures with low hydrogen fraction. Copyright (c) 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.