The laminar burning velocities of biogas-hydrogen-air mixture at different fuel compositions and equivalence ratios were determined and studied using the spherical flame method. The combined effects of H-2 and CO2 on the laminar burning velocity were investigated quantitatively based on the kinetic effects and the thermal effects. The results show that the laminar burning velocities of the BG40, BG50 and BG60 are increased almost linearly with the H-2 addition owing to the improved fuel kinetics and the increased adiabatic flame temperature. The dropping trend of laminar burning velocity from the BG60-hydrogen to the BG40-hydrogen is primarily attributed to the decreased adiabatic flame temperature (thermal effects). The GRI 3.0 mechanism can predict the laminar burning velocity of biogas-hydrogen mixture better than the San Diego mechanism in this study. Whereas, the GRI mechanism still needs to be modified properly for the hydrogen-enriched biogas as the CO2 proportion exceeds 50% in the biogas at the fuel-rich condition. The increased CO2 exerts the stronger suppression on the net reaction rate of H + O-2=OH + O than that of H + CH3(+M) = CH4(+M), which contributes to that the rich-shift of peak laminar burning velocity of biogas-hydrogen mixture requires higher H-2 addition as the CO2 content is enhanced. For the biogas-hydrogen fuel, the H-2 addition decreases the flame stability of biogas fuel effectively due to the increased diffusive-thermal instability and hydrodynamic instability. The improved flame stability of biogas-hydrogen fuel with the increased CO2 content is resulted from the combined effects of diffusive-thermal instability and hydrodynamic instability. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.