An experimental study is conducted to investigate the effect of CO addition on the laminar flame characteristics of H-2 and CH4 flames in a constant-volume combustion system. In addition, one-dimensional laminar premixed flame propagation processes at the same conditions are simulated with the update mechanisms. Results show that all mechanisms could well predict the laminar flame speeds of CH4/CO/O-2/CO2 mixtures, when Z(CO) is large. For mixtures with lower CO, the experimental laminar flame speeds are always smaller than the calculated ones with Han mechanism. For mixtures with larger or smaller Z(CO2 ), GRI 3.0, San diego and USC 2.0 mechanisms all overvalue or undervalue the laminar flame speeds. When CO ratio in the CH4/CO blended fuels increases, laminar flame speed firstly increases and then decreases for the CH4/CO/O-2/CO2 mixtures. For H-2/CO/O-2 /CO2 mixtures, San diego, Davis and Li mechanisms all undervalue the laminar flame speeds of H-2/CO/CO2 /CO2 mixtures. Existing models could not well predict the nonlinear trend of the laminar flame speeds, due to complex chemical effects of CO on CH4/CO or H-2/CO flames. Then, the detailed thermal, kinetic and diffusive effects of CO addition on the laminar flame speeds are discussed. Kinetic sensitivity coefficient is far larger than thermal and diffusive ones and this indicates CO addition influences laminar flame speeds mainly by the kinetic effect. Based on this, radical pool and sensitivity analysis are conducted for CH4/CO/O-2/CO2 and H-2/CO/O-2/CO2 mixtures. For CH4/CO/O-2/CO2 mixtures, elementary reaction R38H + O-2 <-> O + OH and R99 OH + CO <-> H + CO2 are the most important branching reactions with positive sensitivity coefficients when CO ratio is relative low. As CO content increases in the CH4/CO blended fuel, the oxidation of CO plays a more and more important role. When CO ratio is larger than 0.9, the importance of R99 OH + CO <-> H + CO2 is far larger than that of R38H + O-2 <-> O + OH. The oxidation of CO dominates the combustion process of CH4/CO/O-2 /CO2 mixtures. For H-2/CO/O-2 /CO2 mixtures, the most important elementary reaction with positive and negative sensitivity coefficients are R29 CO + OH <-> CO2 + H and R13H + O-2 (+M) <-> HO2 (+M) respectively. The sensitivity coefficient of R29 CO + OH <-> CO2 + H is increasing and then decreasing with the addition of CO in the mixture. Chemical kinetic analysis shows that the chemical effect of CO on the laminar flame propagation of CH4/CO/O-2 /CO2 and H-2/CO/O-2 /CO2 mixtures could be divided into two stages and the critical CO mole fraction is 0.9. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.