Adding hydrogen into natural gas can extend its lean burn capacity, improve engine performance at low load operation, and reduce unburned hydrocarbon emissions at the cost of increased NOx emissions. In this paper, flame propagation of premixedCO2 diluted natural gas/hydrogen/air mixtures under various initial pressures was studied by using a constant volume combustion bomb together with high-speed Schlieren photography. Laminar flame speed and laminar burning velocity as well as Markstein length and flame thickness were obtained for the diluted stoichiometric fuel/air mixtures with different natural gas/ hydrogen fractions and diluent ratios under normal, reduced, and elevated pressures. The results showed that both unstretched flame speed and unstretched burning velocity are reduced with the increase of diluent ratio as well as initial pressure (except when the hydrogen fraction is 80%). Hydrogen-enriched natural gas with higher hydrogen fraction can tolerate relatively more diluent gas.